What exactly does an army reserve immaterial officer do? Type of assignments?

Are you referring to an 01A slot? It's just a job that can be filled with an officer from any branch. It's probably staff work.

How immaterial are you?

Some command slots are actually Immaterial. I've been keeping an eye on such slots for HHC, and Drill SGT units.

In the USAR (I'm assuming you are Active) the thing to really be on the look out for are units that are RFX verses TDA. A good rule of thumb would be to go into FMSWEB in the UIC look up function. MTOE units "could" be RFX (better than 50/50 chance) TDA units will not be RFX.

I don't think we can entirely avoid RFX duty positions over the course of a career, but if you can manage your time in TDA units strategically and knock out PME while there you wont get crushed by RFX OPTEMPO for example while in command trying to knock out CCC.

A dream slot in my view that checks multiple boxes is a TDA company command. I have not found one, and if there ever is one, I suspect it will be Branch Immaterial.