The Optimality box on the Integer Solver tab:

is used to control three tolerances: Absolute, Relative, and Time to Relative. These tolerances control how close you want the solver to come to the optimal solution. Ideally, we’d always want the solver to find the best solution to a model. Unfortunately, integer programming problems are very complex, and the extra computation required to seek out the absolute best solution can be prohibitive. On larger integer models, the alternative of getting a solution within a few percentage points of the true optimum after several minutes of runtime, as opposed to the true optimum after several days, makes the use of these tolerances quite attractive.