The value types the is operation matches, which are the eight the API documents. A value type it may add later is reachable by writing the parameter out with Query.parameter, e.g. parameter("filter_query[categories][is]", "<its name>").
Returns the enum constant of this type with the specified name. The string must match exactly an identifier used to declare an enum constant in this type. (Extraneous whitespace characters are not permitted.)