Beams in bending develop tension and compression in their flanges. The bending capacity of the beam is limited by how much force can be carried by each flange.
The tension flange acts like the string in an archer's bow and the maximum force that can be developed is limited only by the yield stress of the flange material.
The compression flange acts like a strut and is susceptible to buckling before yield stress can be developed. Unless the compression flange is fully restrained then the beam may fail by lateral torsional buckling.
Clause 9.6.1 requires all beams to be restrained at their supports. The strength and stiffness of the restraints are checked using clause 9.12.5.
The restraint is required to hold the compression flange in place and is usually provided at the support by use of the bearing stiffeners and a suitable bearing.
The design procedure for checking a beam section is :