The snow light goes off on the APS-4Cbecause the APS-4C gets its command from an actual snow sensor where the SC-40C’s get their command from the APS-4C.
When the snow sensor’s moisture grid dries out, it no longer sends a snow present command to the APS-4C so it’s snow light goes out. However the APS-4C remains on for the duration of the hold on time set on the front panel dial. While the hold on time continues the APS-4C continues to send a snow signal to the SC-40C’s and hold them on as well. When the hold on time is complete all of the controllers will turn off.