In the supplier bank account creation, what is the difference between Create an External Bank Account and Assign It to This Payee and Add an Existing External Bank Account to This Payee?

If the bank account is already assigned to another supplier site or another supplier, then you can use the Add button to associate it to this supplier.

The application won’t allow you to create two records with the same bank account number/bank/branch combination. Therefore, the Add button is used to associate existing bank accounts to the supplier.