I'm still looking for a good alternative term than "use case" without resorting to a whole sentence trying to ask what the person needs solving or how the thing is going to be used. Doubly so in German.
A mistake I often see myself making is that as soon as I get asked to do something I stop listening and start planning how I'm going to implement the solution. In many cases my preconceived solution will not solve the actual problem, because I've not actually found what the problem is that needs to be solved.
In many cases the requester doesn't really know what they are asking for and will need some prompting to actually think it through. This requires a discussion, rather than a simplistic request to define "use case".