mboxkey_db?

[Shelley Waltz]

I searched, but could not find the relevant infomation regarding
the mboxkey_db database in 2.3.7

My current 2.2.3 install simply has mboxlist_db as skiplist and
this database is /var/lib/imap/mailboxes.db

The new 2.3.7 distribution has mboxlist_db as well as mboxkey_db.
What is mboxkey_db - is this new and for what is it used?  Did
this exist in 2.2.3 in some directory which I need to migrate to 2.3.7?

thanks
S