RMSS or the Rolemaster Standard System is a version of Rolemaster RPG by Iron Crown Enterprises. This tag includes the systems called RMSS and the system before it that are most similar; Rolemaster 1st Edition, and Rolemaster 2nd Edition, but not the version the immediately follows called Rolemaster Fantasy Roleplaying or RMFRP. This tag also includes the republishing of the Rolemaster rules under Rolemaster Classic.
Wikipedia has a good summary of how all the different versions are interwoven. It can get a little complicated, especially after Iron Crown Enterprises went into Bankruptcy and bits of the Rolemaster IP got sold off.
In general though the RMSS tag can be used for all systems that are not Rolemaster Fantasy Roleplaying or RMFRP. RMSS, 1st Edition, and 2nd Edition do not differ enough to make much difference in gameplay.