A sūtra is a single, highly compressed aphorism. The Sanskrit word literally means “thread,” and a sūtra text is a string of such threads: composed in a culture of oral transmission, built to be memorized, and meant to be understood with a teacher or a written commentary. The opening of the Brahma Sūtras, in the translation this Atlas carries, is seven English words: “Then therefore the enquiry into Brahman.” The compression is the design. A sūtra states a position in shorthand; the reasoning lives elsewhere.
“Elsewhere” is the commentary, in Sanskrit the bhāṣya. A commentary unfolds what the sūtra compresses: it explains words, supplies context, cites scripture, raises objections, and answers them. In this tradition a commentary is not a neutral footnote; it is an argument. The commentator is establishing a reading of the whole text, often against rivals who are quoted and examined along the way.
That is why a sūtra of a dozen words can carry a commentary of thousands: in this corpus, the commentary on a single sūtra runs from nineteen words to more than fifty thousand. When the reading feels long, remember which text is doing which job: the sūtra is the anchor; the commentary is the argument.