A set (or list) would seem to work using default values, but I’ve found that it allows multiple values. Is there a way to limit this Attribute to one of those values?
TTBx has no mechanism to enforce value choice, i.e. not accept ‘invalid’ choices.
A practical workaround is to set up suggested attribute values and then set the values via a KA dropdown list. This won’t prevent manually typing a wrong value but if using only the drop-down to set values it should preclude selecting an invalid value.
An agent can easily look for notes that hold any value not in the set of valid values, and take appropriate action – perhaps changing the color or the border of the item to indicate that is requires attention.