Every modulator in a panel has bidirectional communication and reacts to any incoming message that matches it’s midi parameter setup (ie, if you setup a modulator to transmit CC 1, and move the modwheel in the synth, the modulator must react, and vice versa).
Normally, “Called when panel receives a MIDI message” script is used to manage incoming messages that can’t be resolved with a single modulator (patch dumps for example).