V8/usr/src/cmd/Mail/manual/cmds/folder

.ip \fBfolder\fP
The
.b folder
command switches to a new mail file or folder.  With no arguments, it
tells you which file you are currently reading.  If you give
it an argument, it will write out changes (such as deletions)
you have made in the current file and read the new file.
Some special conventions are recognized for the name:
.(b
.TS
center;
c c
l a.
Name	Meaning
_
#	Previous file read
%	Your system mailbox
%name	\fIName\fP's system mailbox
&	Your ~/mbox file
+folder	A file in your folder directory
.TE
.)b