mention what also controls where saved messages go

(info "(mailutils) Mail Variables")
'keepsave'

     Type: Boolean.
     Default: False.

     Controls whether saved messages should be kept in system mailbox
     too.  This variable is in effect only when operating upon a user's
     system mailbox.

OK but mention where they go otherwise.
OK say the are deleted I bet.
And where is the variable that controls where READ messages are saved?
I want them to stay where they were (yes, marked as read), not oddly put in "mbox".