[Dovecot] Whether the THREAD command support cross-mailbox thread?

In the real world, the mails which belongs to one thread could be dispersed in different mailbox, at least Inbox and SENT, so whether the command can search different mailboxes and grap them in one talk with dovecot?

If Not, any other approach to do that?

BTW, what the THREAD=REFS stands for? In the RFC, http://tools.ietf.org/html/rfc5256, i didn't find this algorithm.