What form?

If you regularly get your mail at the box, it's the one you'd want to use.

An exception would be the IRS. If you are claiming a status or credit that depends on where you live, then you would want to use the physical address. Using a POBox when claiming the First Time Homebuyer Credit doesn't work--it automatically goes to exams. Using a POBox and claiming HOH when you are still married is also a no-go. The IRS will assume you are still living with your spouse.

I use a US Postal Service mail box, so if it's for mail I give that address. If I need to provide a physical address for shipping I use my work address. The only people I give my home physical address to are officials such as for a drivers license, police, insurance agent, workmen like for pest control. I do not receive any mail at my home, it is too accessible to thieves.

If someone asks for your physical address they should be able to tell you why to your satisfaction they actually need it.

Hope this answer helps

They usually want your actual physical home address. It depends on the form. If you are having something sent to you, you can use a P.O. Box, unless it is Fedex or UPS who don't deliver to P.O. Boxes.

Any legitimate mailing address. If someone puts a postbox on your garden, the postman will probably use it.