When receiving a product with a serial number, and the option "Existing Serial Number" is disabled, the serial number you enter is created and there is no field for a prefix to the serial number. If the option "Existing Serial Number" is enabled then the list of serial numbers available in the drop down menu will include whatever was previously created, including those with a prefix.
Go to Warehouse / Serial Numbers There, create a serial number. The form includes a field for a prefix to the serial number.
Since the prefix is not always an option when creating a serial number, what is its intended purpose? Or should all situations where a serial number is created include a field for a prefix?