autoexpunge clarification

I'm trying to understand autoexpunge, but the documentation is just not clear. Hopefully, someone can clear up a few questions.

http://wiki.dovecot.org/MailboxSettings says the following:

autoexpunge=<time>: (v2.2.20+) Automatically at user deinitialization expunge all mails in this mailbox whose saved-timestamp is older than <time> (e.g. autoexpunge=30d). This removes the need for expire plugin http://wiki.dovecot.org/Plugins/Expire if you don't care that the expunging may not always happen in time.

What does "at user deinitialization" mean?

What does "if you don't care that the expunging may not always happen in time" mean?

I read the sub-bullets but they just aren't clear. When exactly does autoexpunge occur?

Thanks,

Michael