thanks for the feedback.
Yes, sure, the issue here is how to access a variable from a former stage, not how to define a stage for checking out sources from version control.
For me, as a user, a variable is resolved, if Jenkins replaces the variable with the content during build time. The variable could not be resolved and that's what the error message says? I've applied the different syntax before, and it also did not work. I've also tried this and that, and the ticket description just reflects a current situation, where I did not know the recommended, official way, and I'm very thankful for your hint into this direction.
Yes, the different scopes. I know. This is really a long term pain. Actually, as part of a Pipeline, passing a variable to a later build stage by using a file to read+restore the variable can only be a workaround.