The index section is used to define the domains of the model. The indexes are then used throughout the model when objects like subscripted variables and data coefficients, summations and structured constraints are used. Indexes encapsulate the problem dimensions and make it easy to quickly adjust the problem size.
There are two basic types of indexes; numeric and named. The numeric indexes are used to give each subscript item a numerical value. The named indexes are used to give each subscript item a descriptive name.