Why can't we assign supporting references to the Journal Line Type, Retainage Release, Accrual Basis

For Payables, we have a custom Application Accounting Definition (Copied from Encumbrance Accrual) and a custom Journal Line Definition for the Invoices Event Class.  We have added Supporting References for the Journal Line Types: Item Expense, Miscellaneous Expense, and Retainage in Accrual Basis.  For the Journal Line Type, Retainage Release, Accrual Basis, the fields for Supporting References and Account Derivation Rules are not available for update.  How can we add Supporting References to the Retainage Release, Accrual Basis Journal Line Type?