What is meant by line, dot etc. in mysticism (irfan) and what does the narration “I am the dot under the “ba” signify”?

What is meant by line, dot etc. in mysticism (irfan) and what does the narration “I am the dot under the “ba” signify”?
Concise answer
In mysticism, numbers, letters, shapes etc. are language codes signifying certain occult realities of the world. The ”dot” signifies the real unity of God whose image is placed in man”s heart (mind) and it is for the same reason that the ”perfect man” (insan kamil) has been introduced as the center of the world because he is the vicegerent of God. This saying of the Commander of the Faithful, Imam Ali (AS), ”I am the dot under the ”ba”” signifies the same point.
 
Detailed Answer
1. Numerology and Science of Letters in Mysticism
Numerology and science of letters [ilm al-huruf] has long been the focus of the attention of mystical schools but it is necessary to note that what mystics pursued or intended had been different from what we generally perceived about these sciences. In fact, numbers, letters, shapes etc. are language codes signifying certain occult realities of the world.
When it comes to the nature of the science of letters and mystical numbers, much has been said and is still said and at times, the distorted aspects of these sciences have gradually developed into superstition and magic in their old and new forms. That has occurred when these sciences have distanced themselves from their original and true nature:
حرف درویشان بدزدد مرد دون
تا بخواند بر سلیمی زان فسون[1]
Anyway, there is no doubt that in essence these sciences are highly revered by mystics. Muhyiddin bin Arabi writes in his al-Durr al-Maknun:
"The letters are divine depositories containing divine secrets, attributes, knowledge, decree, qualities, power and intention."[2]
It is necessary to note that numerology is closely connected and intertwined with the science of letters, because from numerological perspective, every letter has a connection with a special number. The numbers are considered to be the spirit running in the letters and words:
نزد اهل خرد و اهل عیان
حرف جسم و عدد او ست چو جان[3]
On the other hand, shapes and geometric symbols are nothing but representation of numbers and their relations in the form of lines and shapes.
Instances of such expressions can be found in mystical literature in which sometimes expressions are used to present an occult or hidden secret so that the wayfarer may get in touch with the realities in a more solid and concrete manner moving gradually from a theoretical and intellectual perception to an intuitive and occult one.  Ibn Arabi writes in the introduction to "Insha Al-Dawa”er":
Given the fact that God, the Glorified, revealed the realities of things as they were and informed me about the relationship existing among these realities through intuition, I wished to turn them into a solid and concrete mould so that those relations may be perceptible to those who are unable to understand them due to a lack of insight."[4]
2. Meaning of ”dot” in mysticism (irfan)
An instance of the secret language is the widespread use of ”dot” in prose and poetic works.  The term "khal" (mole) used in mystical poems is another interpretation of the same "point of unity".
بر آن رخ نقطۀ خالش محیط است
که اصل و مرکز دور محیط است[5]
It has been said in explanation of this couplet: The word "mole" (or dot) signifies the real unity of God and the "encompassing circle" (dowr mohit) refers to the entire existence. Hence, real unity is an inseparable part of God Who is the center of the existence. In other words, the ”mole” which is the unity being and which is on the face on the beloved is inseparable from the truth. Real unity is therefore the center of the circle of existence. The world of power and heavens has emanated from the mole which is the unity. That is to say, the existence of both the world is based on the existence of real unity. As well, man”s talking soul and heart has emerged from the same point of unity which is the center of the circle of existence in both the worlds.
For the same reason, it has been said that the black dot[6] on man”s heart which is the most central part of his existence is the mirror reflecting divine unity:
ندانم خال او عکس دل ماست
و یا دل عکس خال روى زیباست
زعکس خال او دل گشت پیدا
و یا عکس دل آنجا شد هویدا
دل اندر روى او یا اوست در دل
به من پوشیده شد این کار مشکل[7]
 
It can, therefore, be said that the "dot" is one of the secrets of God”s real unity of which a mirror has been placed in man”s heart and it is for the same reason that perfect man has been introduced as the center of the contingent world because he is the vicegerent of real unity of God.
3. Meaning of the narration, "I am the ”dot” under the ba".
The use of the term ”dot” or similar words about letters, numbers etc. aimed at referring to secret meanings is also seen in the narrations. The clearest of them is a saying from Imam Ali (AS) who said:
«ان کل ما فی العالم فی القرآن و کل ما فی القرآن بأجمعه فی فاتحة الکتاب، و کل ما فی الفاتحة فی البسملة، و کل ما فی البسملة فی الباء، و أنا النقطة تحت الباء»[8]
And know that all of Allāh’s secrets are in the heavenly books, and all of the secrets of the heavenly books are in the Qur’ān. And all of which is in the Qur’ān is in al-Fātiah, and all of which is in al-Fātiah is in bismillah, and all of which is in bismillah is in the bā’ of bismillah, and all of which is in the bā’ in bismillah is the dot (nuqtah) which is under the bā’.  “I am the dot which is under the bā’.”
In this narration, it has been pointed out that whatever is there in the world is gathered in the reality of the Quran and that Bismillah al-rahman al-rahim encompasses the entire Quran.
Keeping in mind the above details about ”dot” in mysticism, and also in view of the fact that the ”perfect man” is the existential (takvini) Quran[9] while the universe is the detail of the perfect man (because man is the minor world and the world is the major or greater man)[10] therefore, the Imam”s saying can be interpreted as such: "I am the perfect man whose heart is the mirror of the real unity of God and is the center of the existential Quran because he is the dot under the ba of Bismillah al-rahman al-rahim and "Bismillah… encompasses the entire Quran.
As for the ”dot” being under the ba, it must be said that ”ba” refers to the ”dot” (which is technically called the Muhammadan truth) being in a descending arch.[11]
Sayyid Haider Amuli says about nuqta (dot) as such:  "This truth is called ”nuqta” (dot) because this reality is the first definite entity in the universe which, due to its fixation, has led to creation of other fixed and solid entities.  For example, the dot under the ba which is the ”alif” which had no name, boundary and shape changed into ”ba”.  Ba is the very ”alif” which has been changed by ”dot” under the ”ba”.  This saying of the Commander of the Faithful, Ali (AS) who said, "I am the dot under the ”ba” refers to the same fact. In the same way that the dot under the ba has caused the ”alif” to become prominent and lead to the creation of ”ba”, the Commander of the Faithful, is the dot in the universe which has caused the universe to become evident.  As well, it has been reported that he said:
«لو شئت لا و قرت سبعین بعیراً من باء بسم الله الرحمن الرحیم»
And he said:
«لو شئت لاوقرت سبعین بعیراً من تفسیر فاتحه الکتاب»
These two sayings of the Imam (AS) refer to the same fact. Likewise, another saying of the Commander of the Faithful, refers to the same point.
«العلم نقطه کثرها جهل الجهلاء»
As well, some mystics have been reported to have said:
ظهر الوجود من باء بسم الله الرحمن الرحیم
"Existence has emerged from the ”ba” of Bismillah al-rahman al-rahim."[12]
Here, we shall quote Mulla Sadra who has explained some facts about the dot under the ”ba:
"Among the stations (maqmat) which the "wayfarer" will attain is the station where he observes with his own eyes that all the Quran and, in fact, all revealed scriptures and all the beings are under the ”dot” of the ba of Bismillah. At this time, he sees his own existence under the ba of bismillah. We do not see anything from the Quran except its blackness because we are in the world of darkness. We see the black ink left by pen (on the paper) because the perceiver and the perceived are always from the same genus. The eyes do not see except the colors and the senses do not sense anything but the sensible and the power imagination does not fancy except what can be fancied and the intelligence  does not know anything but the reasonable. As well the light is not perceived except through light. [Thus], the metaphysical entities which are pure light are not seen except with the intuitive power or the inward eye as the Quran says:
و من لم یجعل الله نوراً فما له من نور. من کان فی هذه اعمی فهو فی الآخره اعمی.
And to whomsoever Allah does not give light, he has no light. And whoever is blind in this, he shall (also) be blind in the hereafter; and more erring from the way.
Hence, we do not see with the blackness of our eyes except the black ink of the Quran. And when we go out of this unreal world and migrating from this village whose inhabitants are wrong-doers to God and His Prophets (S) and when we leave this illusive, earthly, formal and rational mode of existence, and get annihilated with our existence in the existence of the words of Allah, we turn from annihilation into existence and from death back to eternal life. Thereupon, we do not see in the Quran any blackness and indeed, what we see is nothing but whiteness and pure light."[13]
For further details about numerology and sciences of letters, see the following indexes:
Question No. 13378 (site: 1127)
Question No. 13383 (site: 4511).
 
[1] Spiritual Couplets, volume, p. 18.
[2] Cited from: Hasan Zadeh Amuli, Tafsir Sidratul Muntaha, in the interpretation of Bismillah al-rahman al-rahim.
[3] Sheikh Bahai, cited (ibid).
[4] فإنّ الله سبحانه لمّا عرّفنى حقائق الأشیاء على ما هی علیه فی ذواتها و أطلعنى کشفا على حقائق نسبها و إضافاتها أردت أن أدخلها فی قالب التشکیل الحسّىّ ... لیتّضح لمن کلّ بصره عن إدراکها و لم تسبح درارىّ أفکاره فی أفلاکها». Muhyiddin Ibn Arabi, Insha Al-Dawa”er, p. 4, Liden print.
[5] Shabistari, Sheikh Mahmood, Gulshan Raz, p. 74, Safi Alishah Publications, 1378.
[6] Referred to in Persian as "Suwaida" which means the black dot, See: Dehkhuda
[7] Shabistari, Sheikh Mahmood Shabistari, Gulshan Raaz, ibid.
[8] Boroujardi, Sayyid Hussein, Tafsir al-Sirat al-Mustaqim, vol.3, p. 118, Ansariyan Publications, Qom, 1416 A.H; Fi Sharh Tawhid al-Saduq Lil-Qazi Sa”eid al-Qummi, p. 32, Ma Fi Ma”nah Yatafawat Yasir.
[9] See Allamah Hasan Zadeh Amul”s works to find details about "Existential Quran" and "Compiled Quran".
[10] Mahdi Rizvanipour, Usul wa Mabani Ta”vil Quran az Didgah Imam Khomeini (ra).
[11] Mustanbat, Sayyid Ahmad, vol.1, p. 177.
[12] و (تسمّى هذه الحقیقة الکلّیّة أیضا) بالنقطة، لانّها أوّل نقطة تعیّن بها الوجود المطلق، و سمّى بالوجود المضاف. (و ذلک) کنقطة «الباء» مثلا، فانّها أوّل نقطة تعیّن بها «الالف» فی مظاهره«10» الحروفیّة و صار باء. و لهذا قال أمیر المؤمنین- علیه السلام- أیضا: «أنا النقطة تحت الباء«2»». و قال: «لو شئت لأوقرت سبعین بعیرا من باء بِسْمِ الله الرَّحْمنِ الرَّحِیمِ. و قال: «العلم نقطة کثّرها جهل الجهلاء». و قال بعض العارفین: «بالباء ظهر الوجود، و بالنقطة تمیّز العابد عن المعبود». و قال الآخر: «ظهر الوجود من باء بسم الله الرحمن الرحیم». و أمثال ذلک کثیرة فی هذا الباب. و قد بسطنا الکلام فی تفسیرها و تحقیقها فی رسالتنا المسمّاة ب «منتخب التأویل فی بیان کتاب  الله و حروفه و کلماته و آیاته». Amuli, Sayyid Haider, Naqd al-Noqud, p. 695, Ministry of Culture and Higher Education, first edition, 1368 (Persian calendar).
[13] Mulla Sadra, Mafatih al-Ghayb, p. 21, Cultural Research Institute.