Let me clarify those mod sources. Easiest way to explain them is to use Pan and Cutoff examples:
1) Alternate - assign to Pan. Now each key press will pan voice - left, right, left, right with offset defined by modulation amount. This is useful to emulate classical "pan spread" feature on prophet synthesizers. On screenshot there are 2 Alternate source: NoteOn Alt, NoteOn Alt.2. Which gives 2 different values on key press.
2) Random - assign to Cutoff Frequency. On each key press cutoff will be modulated by pseudo random value between current point ad modulation amount. Effect is similar to assigning Sample and Hold LFO. But with S&H one holds single note and Cutoff walks. With Random modulation one quickly plays notes and and each note has different Cutoff value. On screenshot there are NoteOn Rand1 and NoteOn Rand2.
3) Fixed (aka Constant, aka DC) - rather less useful. Sometimes it may modify parameter that is not present on UI (sorry, i forgot particular example but i faced this in some VST). Or for example on Prophet 12 synthesizer Fixed (this mod source called DC on prophet) can push HiPass filter into self oscillation, while knob on panel cannot!
Or can be used creative:
modulate Fixed - Volume All - 100 - Inverted
modulate G-env 1 - Volume All - 100 - Direct
Now draw in G-env 1 plucky trancegate pattern.
This way one should not keep balancing volume of oscillators between zero and a little: modulation shifts it relatively.
Potential application of all techniques: pan spread and psychedelic sounds.