If you manage an environment with many thousands of mailbox archives, it can be quite cumbersome in the VAC to scroll though to find the one you want.
A feature was added a long time (pre V6) that “chunks” the list of archives and creates subcontainers for them. The threshold of where this chunking happens is now documented (from 8 SP 1 onwards) :
This means that if you have more than 5000 mailbox archives you’ll automatically get the subcontainers created (eg if you had 16000 mailboxes you would get 4 containers).
You can reduce this number though, to say 1000 and then even if you only manage 3500 you’ll get 4 containers.
It might help you manage the archives a little easier.