I am trying to understand what the different forms of becoming are. This sutta does not explain how they are different.
The different forms of 'becoming' are 'sensual', 'material' & 'immaterial' becoming.
Sensual becoming is when there is 'self-identity' in relation to sensuality. For example, the mind thinks: "I love this delicious food, this is now my favourite good, I am going to visit this restaurant every week". Or: "I love this (sexy & beautiful) woman, she is my wife, I am her husband".
Material becoming is 'self-identity' in relation to a material object or in relation to one of the four rupa jhanas. For example: "I am rich, I have lots of money, this is my big house (which make me feel materially secure rather than give me sensual pleasure); I am now a meditation master because I have attained the 1st jhana".
Immaterial becoming is 'self-identity' in relation to an immaterial object or in relation to one of the four arupa jhanas. For example: "I am famous, I am loved; I have a good reputation; I am important; I am now a meditation master because I have attained arupa jhana".
The scriptures explain:
The craving resulting in new becoming — accompanied by passion & delight, relishing now here & now there — i.e., craving for sensual
pleasure, craving to be, craving not-to-be: This, friend
Visakha, is the origination/arising of self-identification described by the
Ven Thanissaro Bhikkhu mentions in his notes on the Bhava sutta that the Buddha never defines becoming.
The scriptures certainly define 'becoming'; just as the scriptures define the word 'being' or 'satta', as follows:
The consciousness of living beings hindered by ignorance & fettered by craving is established in/tuned to a lower property. AN 3.76
‘A being,’ lord. ‘A being,’ it’s said. To what extent is one said to be ‘a being’? Any desire, passion, delight or craving for form,
feeling, perception, mental formations &/or consciousness Radha: when
one is caught up there, tied up there, one is said to be ‘a
being'. SN 23.2
Thus both 'becoming' & the 'birth' of 'being' appear to be forms of or extensions of 'attachment'.
The Mahanidana Sutta is different to the many other stock suttas (such as SN 12.2 & MN 9) on Dependent Origination. For example, the Mahanidana Sutta does not define 'aging-&-death'; defines 'birth' differently; ignores the six sense spheres; defines nama-rupa & consciousness differently; ignores sankhara & ignores ignorance.
It is rather unusual that DN 15 is called 'The Great' sutta about causation when a number of causes (nidana) are missing or undefined.
Contrary to DN 15, the other stock suttas appear to define 'birth' as the production of the ignorant 'view' of 'beings' ('satta'):
And what is birth? Whatever birth, taking birth, entering, coming-to-be, coming-forth of the various beings in this or that group
of beings based on the manifestation ('play') the aggregates & taking
possession of sense spheres/sense objects, that is called birth. SN
In human bodies in themselves, nothing distinctive can be found. Distinction among human beings is purely verbal designation….For name
& clan are assigned, originating in conventions…Whoever makes his
living among men by agriculture is called a ‘farmer’…Whoever makes his
living among men by merchandise is called a ‘merchant’…that is how
the wise truly see…seers of dependent origination. MN 98
Angulimala [the former mass-murderer], say to that woman: “Sister, since I was born with a noble birth [as a monk], I do not recall that I have ever intentionally deprived a
living being of life”. MN 86
There is the case where an uninstructed person assumes form, feeling, perception, mental formations &/or consciousness to be a ‘self’. That
assumption is a mental formation. Now what is the cause, what is the
origination, what is the birth, what is the coming-into-existence of
that fabrication? To an uninstructed, run-of-the-mill person, touched
by that which is felt born of contact with ignorance, craving arises.
That mental formation of ‘self’ is born from that. And that mental
formation is impermanent, fabricated, dependently arisen. That
craving… That feeling… That contact… That ignorance is impermanent,
fabricated, dependently arisen. SN 22.81
Why now do you assume ‘a being’? Mara, have you grasped a view? This is a heap of sheer constructions: Here no being is found. Just as,
with an assemblage of parts, the word ‘chariot’ is used, so, when the
aggregates are present, there’s the convention ‘a being.’ SN 5.10