The FC4A is your standard Sustain pedal. It sends 127 when engaged and 0 when disengaged. ON or OFF. It is a simple switch.
The FC3A is a Sustain pedal capable of sending all values between 0 and 127 when it is engaged. Combined with the Montage tone generator's ability to interpret these values, it can be used to control the Half Dampering function. This is a very piano-based function where the farther the pedal is pressed the longer the sustain. Actually it is probably better to think backwards here, if the pedal is only pressed a portion of the way, you get a very piano-like response as if the felt dampers have not fully been removed from the strings, so you get a shorter sustain result.

Either can be used with the Montage. The parameter to setup for which you are using is found in [UTILITY] > touch "Settings" > "Sound" > Set Sustain Pedal to the type you are using.

When either pedal is fully engaged (127) the length of sustain is identical and determined by the DECAY 2 TIME and DECAY 2 LEVEL settings within the Amplitude Envelope Generator (AEG).

The magic is not only the pedal but in the tone generator's ability to react to the continuous control messages coming from the pedal.

YOu can read about Half Dampering and other piano-oriented features in the following article: Although written for the Motif XF it refers to the Montage, MOXF, CP4 Stage, S90 XS, etc., etc as well.

Half Damper Function, Damper Resonance, and Key Off Sample

Perfect. Thank you.

FC3A is the pedal for me. 🙂