Transitional year has several uses:
1. The person who isn't sure yet.
2. Someone who has a PGY-2 match (like radiology, ophthalmology, etc.) that requires a preliminary year.
Preliminary years are available in a multitude of flavors: medicine, surgery, pediatrics, ob/gyn, transitional. Family practice first years are categorical, but can be used as prelim for another program.
Most commonly, medicine, surgery, and transitional are used. Some specialties require a prelim year in a general medical area that is more specific to their specialty (like surgery for ENT, urology, and ortho, or medicine for dermatology), but, as you see, there is signifigant overlap. That is why many programs will take prelim medicine or surgery. Transitional years are meant to bridge the gap, especially for fields that transcend the narrow breadth of some parts of medicine (like radiology or emergency medicine (for those EM programs that require a prelim year)). What this means in reality is that, with 6-8 months of electives, this makes the transitional year like being a 4th year med student all over again (with concomitant responsibility, but you're getting paid!). This is why these positions are "cush", and highly competitive - people that have matched into a competitive field are just "biding their time" until their PGY-2 year starts. Logically, think of it - if a program wanted a certain thing, it would be in their program, right (versus leaving it in the hands of the resident, one of whom might do 6 months of variations on radiology, and another who might do 6 months of surgical subspecialties)?
Being a foreign grad, you may find it especially difficult to get a transitional year position.