FFh designates a blank state in memory configuration. Accordingly, as long you have not set another value, FFh will be set automatically to the "free area" when MM is executed.
Note that even if you have set a value other than FFh in the "free area", the ROM mask manufacture process can be completed successfully. (However, please evaluate the value within the MCU operations.)
Supplement: The "free area" you set during MM execution indicates that the area was not needed when you programmed the ROM data to the user area. This is different from the reserved ROM area. Therefore, even if an arbitrary value is set in the "free area" during MM execution, that value will not be programmed to the reserved ROM area. (#104857)