What makes a text a “commentary”? The question is naive enough to allow a complicated answer.

In Sanskrit intellectual history there is not a single word for “commentary” and several words focus on different aspects (`bhāṣya’ for an extensive commentary spelling out aphorisms (MBh, ŚBh, ŚrīBh…), `vyākhya’ or `vyākhyāna’ literally meaning `explanation’ and often used as a synonym of bhāṣya when writing a subcommentary thereon, `vārttika’ originally for a concise commentary in aphoristic form (Kātyayana’s V), later for texts encompassing such form (NV), or written in verses (ŚV) or encompassing verses (PV, TV), `ṭīkā’ for a subcommentary (Bṛ, NVVTṬ…), `ṭīppaṇī’ for a commentary on only specific points here and there and so on, please read more in Preisendanz 2008 and Ganeri 2010). These plurality of words suggests (like the proverbial case of the many words for ‘snow’ in the Inuits’ language) a long familiarity with the practice of commenting, seen as entailing many different approaches to a text (or texts). (Btw: I am not at all claiming that this is unique to the Sanskrit world, don’t start telling me about many Latin words from glossa onwards).

Typically, these texts tend to focus either on the single text they are commenting on or on it together with the one this was, in turn, a commentary thereon (for instance, Vācaspati’s commentary on the NV, taking into account also the NBh and the NS). Another characteristic of such commentaries is that they will explicitly refer to texts of opposing schools, whereas they will just silently reuse texts of their own school, since they feel them as part of their own history, immediately recognisable to themselves and their audience.

Which kinds of texts would one comment upon?
1. In the standard case in philosophy, texts of one’s own school; but also
2. Authoritative (usually religious) texts that did not belong to one’s own tradition, but that one wanted to gain for one’s own tradition (for instance, Abhinavagupta’s commentary on the Paratriṃśikā).

What is the role of commentary in Sanskrit philosophy? It is the standard way of writing philosophy. There was a small number of aphoristic texts which did not present them as commentaries (but which often evoke other views and quote other authors), and starting possibly with Maṇḍana (8th c.) some monographs were written on specific topics, however, the practice of commentaries remained the standard and most common way of doing philosophy, enabling one to write about many topics. A common misunderstanding to be erased is therefore the equation of commentaries with non-original and pedantic work. This was most of the time not the case with philosophical commentaries.

However, the circumstances change with time (as to be expected) and if we look at commentaries post 13th c. the situation looks different.
I will focus on especially two aspects:

1. 1. the relation between text and commentary
2. 2. the relation between commentary and its sources

Concerning 1., many commentaries become increasingly not just about a single text (or a sequence of texts), but interact more with a network of texts (as I have discussed elsewhere in the case of Veṅkaṭanātha’s Seśvaramīmāṃsā, see Freschi 2018).
A very noteworthy case is that of the relation between the Advaitasiddhi and the Nyāyāmṛta. The latter is a very influential text of the Dvaita Vedānta school by Vyāsatīrtha, in some sense we could say that it is the text through which the Dvaita Vedānta becomes part of the mainstream philosophical discourse. How could this happen? Because Vyāsatīrtha took up Madhva’s (the founder of Dvaita Vedānta) central theses, but stripped them of Madhva’s idiosyncratic style and “repackaged” them in the powerful argumentative style of Navya Nyāya. Form is not only a matter of style when it comes to philosophical discourse and this change meant that Madhva’s core ideas and intuitions were now formulated in a strongly inferential form and made a really compelling case for their validity.

At this point, the Advaita Vedānta school could not continue to ignore Dvaita Vedānta. An Advaita Vedānta champion, Madhusūdana, took up the challenge and wrote a detailed response to the Nyāyāmṛta in the form of a detailed commentary (almost line-by-line) to it. This was not the kind of appropriation commentary I discussed above but rather a close rejoinder. At the same time, Madhusūdana needed to invoke his own set of authorities to join the discussion, thus contributing to the network-isaiton of the commentary.

Concerning 2., something I noticed in Veṅkaṭanātha’s commentaries is that they (against what I described above and in Freschi 2014) quote and mention people of Veṅkaṭanātha’s school and silently reuse opponents. Why so? It seems that quotations and reuse have shifted into a way to give prestige and authority to one’s position as part of the school, in a way that the reuse of opponents’ names and direct quotes would not be able to do.