“Fundamentals” is marketed in the US, and maybe some other countries e.g. Canada. “Principles” is marketed in the rest of the world.
Hopefully someone who has seen both books can comment. My understanding, based on discussions like
this one from Quora, is that the two books have the same content but “Principles” is produced more cheaply. (There is some confusion in that discussion, so my understanding may be incorrect.)
I used “Fundamentals” as a student in the 1970s and taught from it in the 1980s. I have not seen recent editions of “Fundamentals” and have never seen “Principles”.
When I use
http://www.wiley.com/ and search for “halliday” I always get “Fundamentals” regardless of my location setting (didn’t try every possible setting).
When I use
http://www.wileyindia.com/ and search for “halliday” I get “Principles” along with some locally-published books based on it (study guides, maybe?).