Are you referring to
Purchasing > Setup > Accounting > Expense account rules?
These rules are setup to help you derive the correct a/c number for expense type of purchase orders. You can specify for example that if the category segment1 on the po line is 'OFFICE SUPPLIES', then the second segment of the a/c number must be 12345.
These rules override the a/c number generated by the a/c generator workflow.
See http://www.oracleug.com/user-guide/purchasing-overview/expense-charge-account-rules for details.