That kind of depends on the book. Comics are published in so many different forms. For American mainstream stuff, you can generally follow this rule of thumb:
OGNs - complete work. Trades - made of collected complete works (ie. pamphlets). Pamphlets with backup stories - main story and backup are separate complete works.
For manga, apply the same logic. Although because the manga industry works a little differently, we're more open to posting large chunks from collected manga, than American trades.
For lit comics, if it's a book of collected short stories by different creative teams, then we'd have to treat each story as a separate work. In the case of a collection of interlinked shorts that are meant to stand as a whole, then we might treat the whole book as the complete work, and the individual stories as chapters.
If you're not sure about a particular work, drop us a line. We're open to making exceptions, based on the history of the book (are the publishers/creators especially relaxed about sharing? was the story published in a variety of formats? is the copyright lapsed/murky? is it out of print or exceptionally old?).