"mid grade" is not a spec, to be precise they spec an octane rating. But they have to write it out as Premium or Regular because it's found that users are idiots.
Even within the US, often dependent on elevation, the octanes are different for midgrade/premium. So say your car specs 89 "regular", but you're up in denver where 89 is considered midgrade or premium, and you're planning on going to sea level, you might want to get the midgrade.