Modelname.Prioropt = 1;

Instructs the mixed integer solver to use priority branching information passed by GAMS through the variable.prior attributes.  If used priorities should reflect knowledge of the problem.  Variables with higher priorities – lower values of the ,prior attribute -- will be branched upon before variables of lower priorities.  This information indicates user specified preferred directions for the internal branch and bound tree search and can dramatically reduce the number of nodes searched.

Problem knowledge may indicate what could be considered first.  For example, consider a problem with a binary variable representing a yes/no decision to build a factory, and other binary variables representing equipment selections within that factory.  You would naturally want to explore whether or not the factory should be built before considering what specific equipment to be purchased within the factory so you would set the priority values lower for the build variables.  By assigning a higher priority – lower value of prior- to the build/nobuild decision variable, you can force this logic into the tree search and speed up computation time by not exploring uninteresting portions of the tree.

Note priorities are not needed and the branch and bound codes used sophisticated branching criteria involving potential of the variable to affect the objective function value.