The mod is the audio signal which is converted from the analog domain to the digital domain pre FX. So the modulation source will be the synth's audio signal pre FX.
As I'm looking at this mod source too, the answer did not offer any clue as to what it's good for in the modulation matrix? ... I've routed it to several destinations but very few of them yield any audible result, some only very very faint...
What exactly is this mod source good for? ... you write that it's the signal after the ADC for the FX, I'm with you that far, but I've only found a use for it, routing it to Pan, as it gives a strange lo-fi like stereo sound.
What frequency is the mod matrix running at in the REV2 anyway?