I guess there are no real limit to which documents it could be applied to, but examples would be:
- Purchase Orders (Based on Order Value)
- Sales Orders (Invoice total)
- Credit Notes (Credit size)
The approval process could work within the existing ToDo process (or my other suggestion of notifications) and would prevent a user from exceeding their authorisation limits. People who have higher authorised limits could approve the document (ie activate to commit it etc).
In this way, you could have purchase order requisitions as a new feature as a user with 0 limit could enter the purchase order document, but not turn it into a committed purchase order (or print it etc).