Modules with lasers
Many optical modules with lasers have a TX laser disabling option. This means that the laser can be turned off by software.
Note: The laser is always on until it is turned off.
User-writable data
Other modules contain user-writable data areas that can be used for identification, licensing and so on. This area can be password-protected.
Read-only data
Writing to a read-only data area will have no effect, and the data remains the same.