Modelname.Optfile = 1;

Instructs the mixed integer solver to read an options file as discussed in the Solver Option Files chapter.  The name of the option file is solvername.opt (ie cplex.opt or xa.opt or osl.opt).  Solver options allow one to manipulate the way solvers work affecting a number of solver functions including choice of the branch and bound tree handing strategies.  The solver manuals cover the allowable options.