Let me begin with a link from the Wikipedia :
In CATIA it looks like this :
We’ll get the above message. To put it simply, it means that CATIA can’t determine the direction for the new surface. And the condition necessary for Join, condition that is at the same time a property, although “invisible”, is that each surface must have its own direction.
During 10 years of design, I came upon such message once – it resulted from being under pressure on a job, and hence, the “quick&dirty” approach, that lead to something like this popping up. I’d tried to combine too many twisted trims.
But how to deal with something like this, when you don’t have the time to start over (although I’m not sure if that’s even feasible…) and the target is the result? – the easiest way to do it, is to convert components of the Join into solid elements and add them up. Here’s an example, with symmetrical application of Thickness operation to components of the Moebius strip :
I wonder how one can bypass the Klein’s Bottle…