As an Inactive Reservist you
A. IRR - Individual Ready Reserve, two categories, the first are guys working toward a Reserve Retirement, taking the necessary actions to make satisfactory years, and the other are guys that were released from the Active service with no requirement to actually affiliate with the Reserves, and so they don't. Technically they are in the Reserves, and subject to recall (see the current Dems bitch about "Backdoor Draft") but are not required to do anything, at the end of the enlistment they will get a 214 (probably) in the mail.
B. Ready Reserve - this is the standard Reservist, affiliated with a unit of some kind and drilling.
IRR -Basically 1. Aren't drawing pay. and 2. Not subject to the UCMJ, with a few limited exceptions, say you were ordered to report to a Reserve Center for a Physical or some other administrative function.
Ready Reserve - You would be subject to the UCMJ while "drilling", or on Active Duty for Training (anythime you are getting paid). However, if you were to be charged with a UCMJ Violation, it has to be acted upon before the end of the "drill" or other period you are going to be paid for, at a minimum charges made and acted upon by the proper authority. And holding somebody over for legal action under the UCMJ is almost impossible.
So if you are thinking about Kerry, he likely was in the IRR in that, go home, we'll call you don't call us staatus. In other words, probably taking advantage of all his First Amendment rights. Not illegal, but definitely not what we want in a President's resume.
Similarly, while Reservists in a non-pay status were able to bad mouth Clinton, anybody on Active Duty or drawing Retired pay would have been violating Aarticle 88.
Aan Active Reservist is a Reservist serving on Active Duty, either long term in a Reserve Training Capacity, with a Reserve enlistment or Reserve Commission, or on some temporary training period, or recalled for say ODS, etc.. An Inactive Reservist is one is not doing anything military wise, between drills, Training Periods, etc.
Probably oversimplified, and some of the terms are often misused in common parlance.
Never thought I'ld ever get any use out of the UCMJ Correspondence Curse.