I'd say that 1, 2 can be used when effect engages more than 1 sample, but there is no obvious spatial separation.
Regarding RM RL LM LH etc meaning... I'm not sure they are still in use. I keep them in SSM set to avoid compatibility issues. To be honest I don't know in what cases they are called by SSA. Maybe in case of AC getting damage. If you will see them being called please share with me.