Receivable SLA Line description for Transaction Description

Hi

When creating accounting, our business user needs to add a description for the AR transaction that should appear with the receivable event. We tried to use the following fields as a source for the line description rule

• Special Instructions
• Comments
• Structured Payment Reference
• Transaction DFF "RA_CUSTOMER_TRX"

But unfortaulty, none of it is available to be used as a source for the line description rule, so please advise if there are any fields we can use at the transaction header which can be used as a source for the line description rule and why those fields are not available to be used as a source