addr:PrivateMailBox_type

A mailbox rented from a private Commercial Mail
Receiving Agency (CMRA).
USPS Pub 28 Section 284: Normally Private Mail Box
identifiers, like mailstop codes and other internal mail
distribution codes, pertain to the recipient and are not
part of the address. "Exception: When the CMRA mailing
address contains a secondary address element (e.g. rural
route box number, suite, # or other term), the CMRA
customer must use Private Mail Box (PMB) when utilizing
a three line address format" (see examples above).