The term secondary set is used when describing fish patterns.
A fish pattern has 2 interacting sets of constraints. These are usually rows or columns, but modern fish variations also use boxes. The 2 sets are equal in size. One of the sets is the defining set and the other is the secondary set.
In a fish pattern, the defining set of constraints only has candidates for the selected digit which are located in one of the constraints belonging to the secondary set. The secondary set has surplus candidates which are not located inside the defining set. These surplus candidates can be eliminated.