The spec for it is currently an Editor’s Draft, so that means nothing here is set in stone because it’s a work in progress. That said, it’s defined as a shorthand for max-lines and block-overflow, the former of which is

The spec for it is currently an Editor’s Draft, so that means nothing here is set in stone because it’s a work in progress. That said, it’s defined as a shorthand for max-lines and block-overflow,