To setup Audit Trail Code prefix and Source Document for posted transactions, use the Audit Trail Codes Setup window(Administration–>Setup–>Posing–>Audit Trail Codes) to enter information for each predefined audit trail code in Microsoft Dynamics GP.
Audit trail codes are assigned to each transaction entry method for each series, and provide a precise record of each transaction, including where its been posted as the transaction has made its way through the Microsoft Dynamics GP system.
Note: Before we use this window, set up source documents using the Source Code Document setup window(Administration–>Setup–>Posting–>Source Document).
Hope this helps!!!