Drop-down fields, also referred to as "set of options" in Novi, can either be locked down to one value or they can allow multiple selections. When creating a Group in Novi with a field that allows more than one item to be selected, you'll want to use the "contains" operative.
It's "contains" because a member could have multiple values in that field.
For example, a member could have values "Item A" and "Item B" assigned to the drop-down field.
Saying that customer's field value "is" "Item B" doesn't make sense, because their value is both values, not just one.
What you can say is that the customer's list of selected class values "contains" "Item B."