[sdiy] Coupling for FM?

[ceme at cox.net]

What are the (electric and (most importantly) sonic) differences between audio FM from a VCO to a VCF with and without a capacitor between?  
On my MG-1, I'm running a VCO through a pot + series resistor straight to the VCF control sum.  It sounds good.
I'm looking at multimoog schematic here and there is a capacitor between the audio wave and the VCF control.  
Why is this and how can I calculate the capacitance necessary for a particular circuit?
Along these same lines, what about VCO to VCO FM and the AC/DC coupling involved?  Why and how?

Chris