What is an Objective-C "class continuation"?

I can't quite figure out what I have seen referred to as an Objective-C "class continuation". Is this / are these…

1. Ivar(s) declared in the @implementation (.m) file?
2. Another name for a class category? (Unlikely, ASFAIK categories cannot have Ivars, period)
3. Another name for a class extension?
4. Something else?

That said...

1. What is the scope, lifetime, and usage case for such a thing?
2. Is this an ARC-specific "feature"?
3. Are there specific runtime, or other requirements for their use?
4. Is this an appropriate place to create an @property, as well? And why would this be a better place for setting ivars or properties than, say, the @interface file / declaration?
5. Why do people complicate discussions by using such specific terminology - that seems NOT to exist in any official documentation (that I could find)?

In question In Objective-C what is the difference between defining something (say a property) in the header file as opposed to the .m file? the discussion touches on this issue, but sort of just clouds the issue further - or at least fails to provide a canonical reference / definition of the term… hence this question.