What regulations cover IRR?

I am doing some research into the IRR. I was looking at AR 135-91 for answers but the scenario I'm looking at isn't explicitly covered. My question is what would happen to someone who has time remaining on a contract, has met their initial MSO, but is needing to go into the IRR for an unspecified period of time? Would their time let on contract just pause or would your time just continue in the IRR towards the completion of the contract? AR 135-91 seems to only cover those SM's that haven't fulfilled their MSO. Is there another Reg that covers this or did I skim over that in the aforementioned AR? Any help would be greatly appreciated. Thanks in advance.