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I've read in the docs:
You cannot create a constraint for a field value attribute (FVA).
As the Category is such an FVA, is there any other possibility to "control" the list of "Category" by Project?
The use case is to have a different list of possible Categories for a Test Document Element dependend on a specific Project.
In most cases, adding a constraint on an FVA field does not work because the FVA field is not editable.
However, Category is a special field that is editable on the content item. You can create a constraint for the Category field.