There are lots of threading inserts, and the several catalog pages I've
looked at to try to understand them don't do a very good job. The
turning inserts seem to make a lot more sense than the threading ones.

So first, can anyone point me to a reasonable explanation of how the
inserts are numbered (threading, not turning)?
Are threading insert brands compatible with each other? For instance,
can I expect to use company A's insert in company B's holder?

And in the too convenient to be true department, can I use a triangular
threading insert to cut internal threads with my insert boring bars?

Thanks,
Steve