What's the actual meaning of "AA/A" on iPhone?

That unit was originally sold in the United Arab Emirates and will have FaceTime outside of it. The /A is the revision code.

(221872)

What is the complete model number?

NOTE:

M = New

N = Replacement unit

F = Refurbished by Apple

P = Personalized.

That would indicate that it was sold as a new unit. However, it does not indicate if the device was previously used. To check warranty coverage, enter the serial number at: Check Your Service and Support Coverage - Apple Support