Filter entities are completely unrelated to all inputs/outputs, as in they do not output anything themselves. They are simply referenced by an output emitting entity (such as a trigger) through a keyvalue called "Filter Type" and they prevent the trigger from emitting any output relating to it's activation such as "OnStartTouch" unless the activator of the trigger passes the filter whether it be a filter_activator_name which discriminates based on name or filter_activator_class which discriminates on entity type (npc_zombie or whatnot).
This is where the trigger references the filter's name: