Does not seem documented. The pattern could probably be triggered two-fold:
- in pre-boot state by the firmware of the ARM Microcontroller (e.g. for the CZ11NIC20 board that would seem to be a STM32F030R8T6TR chip)
- in OS booted state by LED device tree bindings https://www.spinics.net/lists/linux-leds/msg11539.html and userland control settings Index of /doc/Documentation/leds/
Hopefully that will work and provide insight