What are the FS and SU pin names standing for?

[gyscos]

In the IO ports, there are 2 pins connected to GPIO2 and GPIO3: FS and SU.

Does FS mean "Filament Sensor"?
What does SU stand for?

Can they be used safely for that?

[gramresearch]

GPIO is literally, general purpose IO, use it for whatever you want. FS is for filament sensor and that's what I use it for. I'm not sure what SU was supposed to signify, but both are mere suggestions of what they can be used for.