What does the "OPT" build target stand for?
Oh, I found that in the description (should've looked there at once):

The Blinky program is available in different targets:

  STM32F407 RAM:      configured for on-chip RAM
                      (used for target debugging)

  STM32F407 Flash:    configured for on-chip Flash
                      (used for production or target debugging)

  STM32F407 OPT:      STM32F407 with Flash Options Bytes
                      (used for programming)