there are a couple of options.
The customary way that a listserv/mail list is set up is that users who subscribe to the non-digest version get an email each time something is posted to the list. Digest users have the postings combined and sent in a large file far less often.
The frequency and size of the digest are configurable things. For example, the admin can set up the digest so that once a day it sends a file that contains all of the postings for that day. Another common method is that when some number of messages or some volume of messages is received(i.e., when 50 messages are received or when the amount of data is 250K, etc.), the listserv sends out a digest.