Hi Lukas,
If you want to retain the same values of Drop down ,and also restrict the user with certain values ,
Then you can try the below approach.
1. Don't use the context which has all the values required in the drop down . Instaed Create a new context and use it for drop down UI .
2. Populate this context with the values of origial context and while populating restrict the entries .as per your requirement
3. On any entry selection from this DDBI UI , set the related attribute value in the Original Context node
