RS = Road Side (Driver side)
CS = Curb Side (Passenger side)
These are typical geographic circuits of outlets on 120v or lights on 12v. Like a traditional house, separate circuits are set up for loads and to ensure you don't loose everything if there is a fault. It is good to test and map/note what stuff is on what circuit for your rig. That info can be very valuable when issues arise. Also, they might not be where you think. For example, I have outlets on the driver's side on the CS circuit that I would think would be on the RS to make sense.