Last time I checked, which was probably a couple years ago, you couldn't use a PO Box address.
In the U.S., some/all post offices have added the option of real "street addressing." Before, you'd have to use PO Box 123, City, State. But with street addressing, now you can use - say - 45 Main St #123, City, State.
Non-USPS places have had this sort of thing for a while. Some will even "let" you call it "Suite 123" - despite it being a small metal box with not much room for furniture. (I think the "letting" part is more them just not caring, and only looking at the numbers. It's not impossible but I'd be surprised if there were actual laws covering that. Though it could be a factor if something like fraud were being committed. IANAL.)
Googling, it seems like there has been some confusion in the past about what constitutes a "physical address" for the purposes of domain registration, but I don't see anything that suggests that PO boxes aren't allowed.
In the U.S., some/all post offices have added the option of real "street addressing." Before, you'd have to use PO Box 123, City, State. But with street addressing, now you can use - say - 45 Main St #123, City, State.
Non-USPS places have had this sort of thing for a while. Some will even "let" you call it "Suite 123" - despite it being a small metal box with not much room for furniture. (I think the "letting" part is more them just not caring, and only looking at the numbers. It's not impossible but I'd be surprised if there were actual laws covering that. Though it could be a factor if something like fraud were being committed. IANAL.)