A string that associates the payment transaction with a user on your own service.
Mac Catalyst 13.0+
You can use this property to detect some forms of fraudulent activity, typically multiple transactions from different iTunes Store accounts. For example, if you have an online game where each user creates an account to save gameplay data, it's unusual for many different iTunes Store accounts to make purchases on behalf of the same user account on your system. The App Store can't automatically detect that the transactions are related. Setting this property associates the purchases with each other.
Create a string that uniquely identifies the user's account on your service. For example, you can use a one-way hash of the user’s account name. Don't provide personally identifiable information or any data whose disclosure would otherwise be detrimental to the user.
The following code demonstrates one way to generate this hashed data:
Then, set this property before you submit the payment transaction to the payment queue: