A canon is a community’s authoritative textual collection. The Pāli canon is the canonical foundation of Theravāda. It is a large library rather than a single book.
Its three baskets are Vinaya (monastic discipline and its narrative setting), Sutta (discourses and other teaching literature), and Abhidhamma (systematic analysis).
The Sutta Piṭaka contains five Nikāyas: long, middle-length, connected, numerical and minor/miscellaneous collections. MN 118 means discourse 118 of the middle-length collection.
A commentary or manual can be deeply influential without being part of the canon. Buddhaghosa’s Visuddhimagga is an example.
This page is an orientation to the work or collection, not its full text.