drnuncheon
Explorer
Going by a particularly flavorless, rules interpretation:
If they're the same as the entry in the Monster Manual*, you can call them with the Gate.
If you had to do something more to create their stats than just open the book, they're a unique being.
So, pretty much like Dingo said - if they're advanced in any way (i.e. have spearated themselves from the "common throng" of pit fiends), then they're unique.
* - Tarrasque excepted, of course.
If they're the same as the entry in the Monster Manual*, you can call them with the Gate.
If you had to do something more to create their stats than just open the book, they're a unique being.
So, pretty much like Dingo said - if they're advanced in any way (i.e. have spearated themselves from the "common throng" of pit fiends), then they're unique.
* - Tarrasque excepted, of course.