I was interested in this at one time.. Check the Critical Care boards as we have some EM guys over there that could give more info. Here's my understanding...
The CC boards used to be entirely under the purview of the ABIM but the political landscape has changed to where the ABIM and ABEM had supposedly agreed to co-sponsor the CC boards, meaning that EM physicians with a CC fellowship could sit for the boards. That's still in the works from my understanding even though it looks like it will indeed become a reality "at some point" though there's been no specific date or timeline to my knowledge. So, EM docs who did a CC fellowship used to, and probably still are... sitting for the European CC boards (EDIC) to facilitate hospital privileges to work in the ICU but it's always been sort of up to that particular hospitals discretion. I think everyone agrees that if/when this ABIM/ABEM co-sponsoring of the CC boards goes through and EM docs can actually sit for the American CC boards, things will become much easier.
As far as the specific type of work environments, MICU vs SICU, I have no idea, but the CC board would be a good place to ask.