Res 42 is required for the 4 states that require an AOA approved internship for licensure to practice in that state. It doesn't matter where you went to med school or residency, but if you want to have a full, independent license to practice medicine in one of those states, you will need to either (a) complete an AOA internship or (b) apply and have a res 42 waiver granted.
BIG CAVEAT --
Now that the single accreditation system (aka "the merger" is coming into effect), no one really knows what will happen with respect to Res 42. It may be a moot point, as AOA internships will no longer exist in their current form. The AOA is working with the four states who currently require a Res 42 waiver. This requirement may end up being dropped which IMHO, would be a great thing.
For the heck of it I applied today for the ACGME waiver. It was actually a very short online form. It may never matter but since I just got my permanent license in SC I figured what the heck? Took 5 minutes. Looks like AOA will send confirmation paperwork to my program to verify and likely a rubber stamp approval.