[Dovecot] Difference between Indexing and Rescan in FTS

I've had squat running on dovecot 2.0 and have been updating all users mailbox indexes nighty via cron with this command:

doveadm -v search -A text xyzzyx

I've just updated to 2.1 and I'm migrating to lucene indexes, but reading the documentation I'm having a hard time understanding the semantic differences between indexing and rescanning.

If I were to continue to run an all user all mailbox index every night, would that be appropriate?

Would running this every night avoid the need to ever rescan?

Should I run rescan instead of index?

Should I run both rescan and index? In which order?

Best Regards,

FredK