I honestly don't know how it works.
I would guess that it is included into the client.exe file?
I highly doubt that the working is based on .upk files.
And no, using an exe from a region (say, NA/EU) in another region (say Taiwan) won't work properly.
It will do weird things, including (but not limited to):
- Download gameguard
- Download xigncode3
- Crash for no reason
- Say that some file is missing and crash
- Say that the version of the xml.dat file is incorrect
Doing a file repair will remove the client.exe file