Document Change Rules are helping when you want to define fields which should be maintainable after initial entry. Sometimes you will need to change the document header text or other field on later stage through entering to a document in change mode.
IMG Path: Financial Accounting (New) -> Financial Accounting Global Settings (New) -> Document -> Rules for Changing Documents -> Document Change Rules, Document Header
When you open the transaction you will get the lest of the current header rules. To create new Header Rule, click on the New Entries Button.
Maintain the options as you required and save it.