My understanding is that this tracking ID was a replacement for apps being able to obtain the devices serial number. The tracking ID is unique per developer (edit: not true, it is unique per device, so multiple devs can build a profile on you) and can be reset by the user at any time.
I use analytics in my company’s app and I get my team to send me their IDFA by downloading a free app from the App Store. The IDFA in my analytics is the same IDFA in the 3rd party app.
The IDFV is unique per developer; the IDFA is unique per device.
The device user can reset their IDFA at any time, and if the user disables ad tracking then the device returns 00000000-0000-0000-0000-000000000000.