what is the document change rule?

The functionality Document Change Rules configured in the system maintains the information relating to what fields can be changed? and under what circumstances?. As you are already aware, SAPs document principle does not allow changing the relevant fields once a document is posted; any changes can only be achieved through Reversal or additional postings. Fields such as company code, business area, account number, posting key, amount, currency, etc., can never be changed once the document is posted. However, SAP allows changing some of the fields in the line items such as payment method, payment block, house bank, dunning level, dunning block, etc. These can be changed document by document or by using mass change for a number of documents in a single step. The changes to master data are tracked and stored per user for an audit trail.