One method of modelling the problem consists in imposing that either one job is before another one or vice versa for all pairs. So exactly one of the two indicated inequalities must hold. How to model this disjunctive constraint? We make use of an integral variable x related to the two jobs and a very large constant M. Then we impose both of these constraints, but if x is 0 then the two constraints become these two constraints and you see that the second one is redundant. Whereas if x is 1 then the first one is redundant, and the second one is in force.