Slitherine

What do these symbols stand for?

PD = portable defenses. Click on it to toggle between buying troops with or without stakes (portable defenses).
S = Dismountable troops. Hover over the S to see the dismounted troop classification.

PD = Portable defences, basically stakes. to buy them you must click on the option in the DAG this will cost 1pt more per unit. To use them you must right click the unit and it will give you the option to plant stakes.

S = ? but basically in the DAG if you highlight the S it will show you what the dismounted version of the unit is, on the battlefield I would assume you rightclick on the unit to dismount, which must be done after deployment but before they move, but I must admit I have never tested this option myself.