RST 2 is usually used to write to the current channel, usually the screen.

Internally, RST 2 calls the routine in the CHURCHL vector, returning with all registers preserved unless an error occurred.

Implementing custom channels

Because the rom code uses the CHURCHL vector you can easily replace the current output channel with your own code.

My Teletext emulator does this to allow code to write to the new screen using RST 2.