Primary will be where the warranty is located. The 500 would be North America, the 555 would be Japan and the 502, I think might be China and Europe, but not sure.
As I understand they're all the same basic product, and the different model number is assigned to show which region it's intended for. Knowing the region is important because, as Jan said, that's where the warranty coverage is going to come from. So a US user buying a 555 or 502 would have no warranty in the US; a NZ customer buying a 500 or 555 would have no warranty coverage in NZ, etc...
Good thing is they all have same shooting modes no matter the location. For this price, another great feature. Barry HPX bootcamp in Aus next year? Actually forget it, Ill need a competitive edge