section modulus is a measure of the strength of a beam. The more the section modulus the more is the strength.

section modulus of any section is the ratio of the moment of inertia to the distance of extreem fibre from the neutral axis. plastic section modulus is the section modulus when the cross section is subjected to loading such that the whole section is under yield load. numerically it is equal to the pdoduct of the half the cross section area and the distance of center of gravity of tension and compression area from neutral axis

The bending stress in a beam is inversely proportional to the section modulus.

Plastic Section Modulus about the element local y-direction

Torssional section module