if your TY program has an IM program at the same institution, you will get 1 month of credit for each month that you rotated on an IM service, and were treated just like the IM PGY-1's. This might include the ED, or the ICU, or other services where PGY-1's are all treated the same. You will not get any credit for peds, surgery, OB training -- although, sometimes a PD will be willing to give you 1-2 months of credit for these types of blocks, counting them as "electives".
If your TY program does not have an IM program, then you are likely to only get 3 months of credit maximum.
Hi there. I did a transitional year at an FP program that wasn't associated with IM and jumped into IM after that year was completed. Got 6 months of credit for my year of TY, which is the absolute max you can get joining IM from another specialty. Once I decided to do IM, I purposely scheduled IM rotations - nephrology, cards, ID. Got credit for radiology, ER, and another rotation (but I can't remember which one) as well. It helped that my IM PD requested the max of 6 months, which may have been because I received excellent evaluations after starting IM.
As for "being treated the same" as an IM rotation, my FP inpatient months were probably just as intense as any IM I have done (and didn't have much OB or peds at all those months). At least that's my opinion.