The NPC was asked to change the term "Rho Chi" by the Rho Chi fraternity, which is (I think) an honorary pharmacy fraternity. Since then, many (but not all) schools have made the move to switch the Rho Chi name, but since there was no standard, there are about a hundred different names for Rho Chi's. Here's what they theoretically stand for:

Rho Chi: Recruitment/Rush Counselor
Rho Alpha: Recruitment Assistant
Rho Gamma: Recruitment Guide
Sigma Rho Chi: Sorority Recruitment Counselor
Pi Chi: Panhellenic Counselor
Gamma Chi: Greek Counselor
Pi Rho Chi: Panhellenic Recruitment Counselor

And just about every other variation possible. The NPC is now pushing the term Sigma Rho Chi for the official replacement.

I have no clue what the Sigma in Rho Sigma would stand for . . . maybe they just made it up?

I'm pretty sure you're talking about SDSU (sorry if I'm wrong), but if so the last night of sorority recruitment is also called pref. It's basically the last nite, where you can only go to 2 houses (at our school). After that you fill out your pref card (after talking w/ your sigma rho chi if needed), and find out the next day about your bid :)

We also have "Pref" about a week after recruitment is over where, like fingerbang said, a sorority and fraternity have a dance/exchange together.

According to NPC it stands for Sorority Recruitment Counselor.

Rho Sigma at MTSU (I was told) means recruitment sister.