So what's inside?
new units, known to the public (and modders of RA2), but arranged otherwise
changed game rules
new systems: RCS and new mind control system.
Version A: new units added, changed gamerules, RCS (+ mind control system)
Version B: new units added + changed gamerules
C: new units + RCS
D: new units with slightly changed gamerules (such as decreasing Boomer's cost ;-))
E: only new units (without changing anything)
For a nearly complete changelist, you can look here:
The RCS stands for Realistic Combat System, its mission is simple: to make RA2 look more realistic, for example by enabling constant fire of infantry or inaccurate shots by artillery and big-shells units.
There's also a new mind-control system. Originally, as maps, it wasn't planned to be featured in mod. PSI units can control up to 99 units, but it takes much more time to recover and regain mind control abilities after each single overtake of a unit.