If you want to know everything about gates and triggers, I can't think of a better description than
this article by Gordon Reid.
"V-Trig" means "voltage-trigger" and "S-Trig" means "switch-trigger". I've also heard of "S-Trig" being described as "Short-Trigger", as the switch would short-circuit a voltage to ground potential.
Basically, it refers to the means of triggering an envelope generator in hardware:
A V-Trig envelope requires a positive voltage at its input to determine the gate 'ON' time.
An S-Trig envelope requires a normally-open switch (infinite resistance) at its input to close (zero resistance), to determine the gate 'ON' time.
S-Trig is less common in the hardware world, with Moog as the most famous user of the standard. This could be a historical 'accident' based on Bob Moog's early development of an ADSR envelope generator; he asked
Herb Deutsch to go and buy a doorbell push button to use as the trigger!
There is no concept of electrical resistance (or a switch) inside the Voltage Modular world - EVERYTHING IS A VOLTAGE.
Therefore, whenever you see a socket labelled "S-Trig" within VM, it is always for historical cosmetic reasons.
For example, the
921 Oscillator in the VM900 Collection has both S-Trig and V-Trig inputs to the clamping point trigger. Either can be used as a V-Trig input, though the V-Trig responds to positive-going voltages, while S-Trig responds to negative-going voltages.