The United States Postal Service maintains a database of every deliverable address in the US.
This data is collected and maintained by their letter carriers.
As part of their regular processes, letter carriers identify addresses as either primarily residential or non-residential delivery points. That information is recorded as what the USPS calls their "Residential Delivery Indicator" (RDI).
There are times when the letter carriers record addresses that are valid addresses, but for some reason are not currently collecting their mail. This is recorded as a "Vacant Flag". It is important to understand these addresses could receive mail, but are not currently receiving mail. This is different from addresses that are not considered valid for receiving mail, for example addresses that might be assigned to new developments where buildings have not been built or are not yet occupied.