The word conventions of the DFARS PGI follow those in the DFARS, which in turn follow those in the FAR. The imperative word "shall" means the action must be taken (unless a waiver provision is available). The term "should" means "an expected course of action or policy that is to be followed unless inappropriate for a particular circumstance" (FAR PART 2). This can be understood as mandatory, except the contracting officer has latitude to determine if it's applicable for a particular acquisition situation. The term "may" often provides optional alternatives, e.g., "Evaluation factors may include—..."