A private mailbox — a PMB — is a box rented at a commercial mail-receiving agency rather than from the postal service, written as a PMB or # number after that agency's own street address. On paper it looks like a suite in an office building, because that is the form the postal service requires.
What Spaw answers
is_private_mailbox is the flag and address_type reads private_mailbox. At +60 it is the heaviest of the four delivery-point weights, though not the heaviest weight in the address product: an address the deliverability check reports undeliverable carries +80.
The detection is the interesting part: it comes from the way the postal service requires such an address to be written, not from a licensed database of agencies. A PMB 214 or a # 214 after a street address is the tell, and it is a rule about spelling rather than a lookup.
The limit that follows from that
A private mailbox written without its PMB number is indistinguishable from a suite. Where the licensed deliverability check would answer the question outright, this service does not run it — is_cmra stays null — so the flag catches the addresses that are written correctly and misses the ones that are not.
A PMB is not a lie. It is a mailing address, useful and legal, that says nothing about where anybody lives.