Operating over part of a set

Ordinarily one operates over each and every element of the reference set that an item is defined over.  Thus in the case

NX(II,J)=4;

Each element of NX associated with every element of II in interaction with every element in J is operated over.  However, there are cases where one wishes only to operate over part of those cases.  In such a situation GAMS can be commanded to operate over part of the set through defined subsets, conditionals or tuples.  Each case will be covered below.