Composite beam design works on a member-by-member basis by creating an Oasys Compos file for each composite beam that you are checking. It includes the load cases and deflection criteria that you specify, along with the beam and slab properties, from the GSA model.
Steel design works on a member-by-member basis and checks each member for strength and/or serviceability using the combination cases specified. Sections are drawn from the Steel Section Pool that the member refers to and these serve as possible candidates for the design. If a member fails a check, it is assigned a fitter section from the pool. Sections are ranked based on the Design Objectives.