lmtp_save_to_detail_mailbox

[dovecot at ptld.com]

lmtp_save_to_detail_mailbox:
" If the recipient address includes a detail element / role (as in user+detail format), save the message to the detail mailbox. "

Im not understanding this, what is the "detail mailbox"?
I tried testing this feature by setting "lmtp_save_to_detail_mailbox = yes" and sending an email to user+test at example.com.
The email still ended up in the default inbox folder.
So what does lmtp_save_to_detail_mailbox do or is there another setting that also needs to be set to use this feature?