Hello Tiger Lo,
1. If system already set FSEC to 0xFF (Protection, as I check the manual, it seems ca only erase all & can't erase
block ), there is any other method can archive let 16 byte flash as data that I can write some run time data as
parameter even when system power off.
Answer: FSEC is used for security, not the flash protect. You can find more details from the KL03 reference manual:

You can find, flash security is used for the programmer interfaces, if you use the internal code to operate the flash, it won't be influenced.
You can use your code erase and write the flash.
2. Can I set the security bit to disable then do the 16 byte operation when it complete then set the security bit enable ?
Answer, security has no relationship with your 16 byte operation, you can use one flash sector as your own data store area.
3.can I use the program once file as data to store ?
Answer: No, program once area is just can program one times, then you can't modify it. So I suggest you still use one flash sector as your own data area.
Wish it helps you!
Have a great day,
Kerry
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