Traditionally STAAD.Pro performed code checks based on single analytical members (i.e., single members between two nodes). This implementation assumes that the design inputs (except forces and section properties) remain same throughout the length of the member. Physical member design on the other hand allows you to group single or multiple analytical members into a single physical design member for the purposes of design to NZS 3404. Thus the some of the design inputs related to member strength (unbraced lengths etc.) varies along the length of the member.