meaning - What does it mean by "the round-trip problem"?

A round trip in real life is say from home to work and back home again. A round trip in code generation is from diagram to code and back to diagram again, or from code to diagram and back to code again. Often you do not end up with the same diagram you started with, or the same code you started with, after going there and back again. It's a real problem.

Trivial example: take some well commented code, generate diagrams from it, generate code from the diagrams - where are the comments?