Have you tried it out? I doubt there is a general accepted definition. For me it is just more capable of deep reasoning/handling complexity. Still can mess up, still does not count as strong AI - but a level above Opus and co.

Could you give me an example of a similar task you asked Fable and a different model to do where Fable did a better job?

I have a hard time getting models like GLM 5.3 to not perform on my tasks but I might be biased.

Changes in a complex codebase. Opus can do it, but needs more handholding. Making the plan with fable and let opus implement it worked out well.

That's actually not quite what I was asking for.