When submitting your app for review in the Apple store it will ask you the question:
Does this app use the Advertising Identifier (IDFA)?
The Advertising Identifier (IDFA) is a unique ID for each iOS device and is the only way to offer targeted ads. Users can choose to limit ad targeting on their iOS device.
Ensure that you select the correct answer for Advertising Identifier (IDFA) usage. If your app does contain the IDFA and you select No, the binary will be permanently rejected and you will have to submit a different binary.
Yes, Unity Ads does use the IDFA.
Once you select “YES”, there are three more questions Apple asks about the IDFA. The answers to these questions are: YES, YES, NO.
The answer was taken from this post in the Unity Forums