Assistance with Creating a DFF for Clearing Account at the Customer Level

Summary:

I would like to create a Descriptive Flexfield (DFF) named "Clearing Account" at the Customer level (Profile, Account, or Site). The purpose of this account is to override the "Unapplied Cash" account on the Receivables receipt in scenarios where the receipt is an "Unapplied Cash" event (No applied to transaction)

Currently, the "Unapplied Cash" account is defaulted from the Receipt Methods and Classes, which are locked. Instead, I want this account to be derived in SLA (Subledger Accounting) based on the accounting value specified at the Customer level.

I have the following questions:

1- Which DFF or field at the Customer level can be used for this purpose?