modular cutter spaces

[SteveS]

I'm looking at the specifications for the Modular Cutter, and I can't figure out how to account for the space. Here's what I can figure out, with dton spaces left:

50 SL Hull
12 Power & Propulsion
? Occupancy 5 RCS
0.5 Cargo
2.5 Basic Bridge
1.0 Life Support
0.5 Small Craft Add-On
30 Module

The items I recognize add up to 46.5 dton. I don't know what "RCS" stands for, but I also see "RS" in the "Occupancy" section; stateroom occupancy is just one to two per stateroom, without an abbreviation. Does the "RCS" account for the missing 3.5 dton? Or am I overlooking an implied component?