Slitherine

This could be my last question : in the DAG, I can see some special abbreviations like "PD" for "portable defence".

I know that the "S" is about dismountable mounted troops (at least). But what does that "S" means ?

I don't know what the "M" means either.

I'm not sure but I think the "S" means "Substitute"

Where have you seen the "M"?

S = alternative troop type (maybe 'switch'??
M = mixed BG (two troop types in one unit)