Here is how I understand it, someone correct me if this is off.
REC can be earned in Arena Commander in certain areas, such as racing, swarms. Not in individual, it must be competitive. Those REC can be used in the Electronic Access store for renting stuff for (7) 24 hour periods. The stuff can only be used in Arena Commander, not in the PU (current exception is weapons, which can be used in the PU, but this goes away in 3.0 as noted by
@Blind Owl)
aUEC (and eventually UEC) is earned for missions in the PU. These credits are used for in game purchases. I've not seen a store in game where I can rent items.