Hi 123gmax
FSEC is used to secure the SWD interface, after it is secured, you can't read and write the code through the SWD, if you enable the mass erase, you can can do the mass erase through the SWD, it is used to protect the chips app, to limit the app is read out through the SWD.
But it won't influence your own code, you still can use your own code, or the secondary bootloader to operate the flash, the FTFC comander still can be used in the chip's own code.
Wish it helps you!
If you still have questions about it, please kindly let me know.
Best Regards,
Kerry