is it of importance when designing a beam to give/find the plastic modulus, elastic modulus or you can just use one of them.+ the deflection compared to the permissible deflection(l/360).
These values are published, so it should be of no consequence to supply either value. However, in order to determine deflection, you will require the moment of inertia (Ix or Iy) and the modulus of elasticity (E).
Depending on the section and the various b/t ratios for the elements; this may determine if you can use Sx or Zx for the elastic and plastic section modulii.
With plastic design as opposed to elastic design (both limit states can be applicable) you normally use the Zx and the section (in Canada) has to be a Class 1 section.