The Source field has several uses:
You can use it to identify where a transaction originated. For example, if you are entering a transaction for sales, you can enter "Sales" in the Source field (although filling in this field is optional).
You can enter the word "ACCRUE" in the Source field if you want the system to automatically enter a reversing entry when you post this transaction.
If you pass off transactions to General Ledger with General Ledger Passoff (Function GLP) or with the Daily Journal Report (Report RDJ with Option Z), the system automatically enters the Source, as follows: Function GLP will enter PR in the Source field for payroll transactions or AP in the Source field for accounts payable transactions. Report RDJ will enter RDJ in the Source field.