Serial Number protection option reqires your customer to enter pair - Serial Number and Customer ID. Workbook is opened only if both strings are entered correctly.
Serial Numbers item in the left pane shows list of the Serial Numbers defined for this file.
Use Add, Modify, Delete buttons to modify this list.
You can combine this option with:
Option Registration is required after allows to provide evaluation period for your workbook. This is not the same as Combine with Trial Period option! "Combine with Trial Period" option defines working period of this workbook. When this period expires - there is no way to unlock the workbook.