what is the digest in iforum used for

Can anyone tell me what the 'digest' functionality in iForum is used for? And whether it IS used? 

It was carried over from NewBB.

It's one of the 'Topic Options' at the top of the page. And there is a spot in the module's control panel to view Digests. Beyond that, I'm not sure - I don't see any notification options related to Digest.

There is an icon for digested topics on the forum index page, too.

I'll remove that feature from the new version of iForum I'm preparing. It can be added later if there is enough demand for it.

As a matter of fact, I didn't really understand the use case, but I get it now : this could be part of a site-wide functionality that is then used for a weekly//monthly newsletter 'things that you missed', or it could be in the welcome page of the site : activity since your last visit.

I see, it's something that will not be present in the first iteration, I'll be focusing on the forum part itself, but it's something to be listd under 'opportunities for new modules'

I would agree with that - there are other platforms that allow you to select your notification options - every message, or a daily/weekly summary. The manual action of 'digesting' a post would put more in control of the forum admins on what is included.