contains is used to check a list of values, which is given on the left of the operator, against a single value on the right.
A person can have multiple nationalities. If you want to include US tax documents if one of those nationalities is as a citizen of the United States, attach this condition to those documents:
p is Personp.nationalities contains us
The list of nationalities of the person in this case should include
us , meaning this condition evaluates to true and the documents to which it is attached will be included in the case.