I look at it much the same as the car world. There are three levels involved.
Stock - just as it came from the manufacturer
Modified - throw a few different parts at it (handguards, wheels, seat covers, grips, etc.)
Custom - Going completely out there and doing something completely different.
The problem with it though is that modified, in the gun world will scare people off. Who knows what someone hacked on to get some part to work in it. And thats why I think a lot of people use custom. It sounds better. Even if it really isn't.
I haven't seen any other AR's like mine but I'm sure its not entirely unique. I would consider it a custom AR though since it was assembled by me using a very specific parts list. And just from that, it fits this definition of custom.