Ratz, just when I thought I was getting it.
Ok  the side chain switch is only used in a buss situation where tracks are sending a signal to it?
Why have the switch at all then?
Because ANY "Send" sent to the buss, is affected by the comp on it no?.
I thought the switch was a way to get a send/return happening within a track.
IOW how does one use the side chain switch?
 

I also see there are some of the buss comps are missing in the track PCh.
IOW I can only use 2 comps in the original track and no repeats.
The PC4K channel and the PC76 U type. 
So I guess here you would use the 4K (that has no wet dry) for gentle and preliminary bringing down the stray transients and the 76 set to a heavier comp and using the wet dry to mix in for the side comp?

The purpose of the side-chain is to allow one signal to trigger an effect on another. It has nothing to do with whether a plug-in is on a track or a bus. There is a video on this page that has a couple of examples of how to use a side-chain.

What the Sidechain does is allow a different track to trigger the compressor. The simplest example I've seen is the radio announcer one.
 
You have a channel that is running your music. You put the compressor on that track. Then you have another channel that the DJ/announcer is speaking into. You set the side chain on the compressor to accept the signal from that track. Now when the DJ speaks it triggers the compressor which turns down the music as he talks. When he stops talking the music goes back to it's normal level.
 
Also in dance music they'll use it to have the music levels go down every time the kick drum hits creating the "ducking" effect. Used subtly it can be used to get your bass guitar out of the way of the kick drum in your mix.
 
So essentially sidecahining is not what you want to do here. It is a cool technique though.

Regardless of the name of the plug-in, PC modules all work in tracks or buses.

I think there is a limitation to how many instances of a module you can use in X1. In X2 you can toss them in willy nilly. I forget though.
 
Oh and don't forget that you have other compressors like the Sonitus one. Those have to be added to the FX bin though.

Right thanks
Forget the side chain idea.
I was confusing it with the send return/blend NY Comp idea
Sorry. I do know how to do the ducking thing but the two got related in my confusion.

What sound are you trying to get; or, is there a specific problem you are trying to solve?

It's a compression/balance/clarity thing.
My tracks are sounding over compressed.
This is due to compression on the original track, often several times, in order to get the quieter passages up.
EG mumbling, level differences, dropped endings and lack of sibilance. IOW a word like "Noose" comes out like "New".
With heavier compression the "S" and other quieter passages come out fine but the rest of the track is hammered.
In theory, with NY or parallel comp, this should work out with the the best of both.
Yes, I can also do volume automation or gain automation on the individual parts, but I'm working on getting on top of my bad compression habits first and then saving automation for the end.
Does this make sense?

You're leaving out the parallel in parallel compression.  The technique is basically running the same track both nekid (w/o compression) for the hit while using a heavily compressed signal to add body/sustain to the rest of the sound.  The best of both worlds.  SOS had a rather long but nifty article a few months back - it ought to be off subscription by now.
 
The easiest way is to clone the track, squash the bejebbers out of it, and then adjust the volumes between the two tracks to taste.  If you want to do the same for multiple tracks, like drums, use a send (that's one of the things the gods put sends on tracks for).  Use a send to a bus w/ the squashed comp on it, but leave the individual  tracks themselves going straight through to the master.  They supply the dynamic part of the drum sound, then bleed in the bus comp to fill up the rest of the sound field.
 
Actually, it is easier to use a comp w/ a blend knob.  Send the signal of whatever through it, squash it and then use the mix/blend knob to restore the dynamics.
 
@