This force is labeled P' and points to the left. The first mass m1 exerts a force P on this mass, m2. This particular example is stated in terms of weighing a fish in an accelerating elevator. It is also fun to think of weighing yourself in an accelerating elevator. The forces acting on the fish are shown in the free-body diagram.
T is the tension supplied by the scale.
This is the value the scale reads. We may call it the apparent weight of the fish. This fish is moving along with the elevator. In this diagram we have taken the acceleration to be up so it is positive. While the elevator accelerates up ward, the apparent weight of the fish is greater than its true weight, mg.