Grouping in views: Why do I not see all existing attributes?

Hello

I created a specific view for my artifacts and I want to group them by an attribute. But: I do not have all of the used attributes available in the selection in the grouping-menu. What is the criteria, that an attribute would be shown in the list?

In my example I have attributes 1, 2 and 3 in the view. But in the list in the grouping-menu I just see attribute 1 (for example) and a lot of other also existing attributes; but I do not use them in the specific view. How can I get all attributes to choose in the menu?

We use Doors NG Version 6.0

Thank you for your help!

Greetings

Sandro