Basically a digest is an email sent by the server that contains posts from subscribed threads.
The way vB did it was you would get one email for each thread and each post was completely there, on a daily basis. No new posts, then no email. I would call that a FULL and also a SEPARATE digest.
The way IPB currently does it is to send you one email with a highlight from just a few posts for all of the threads that you've specified to send emails. I would call that an INCOMPLETE and also a BULK digest.
SC