What are the properties of the composite section?
When a beam is loaded, the bending resistance is provided by a couple of internal forces, one in tension and one in compression, times the lever arm between them. For the regions of positive moments the tension is provided by the plastic stress of the steel beam, and depending of the location of the plastic neutral axis, the compression is provided by the concrete slab. For the negative moment regions, the tension is provided by the rebars in the slab, and the compression is provided by the steel beam. To satisfy the equilibrium, both forces balance each other, this is, C=T. The figure below shows schematically the plastic stress distribution for both cases.
Conservatively, the AISC allows to consider the steel section alone (non-composite) for the negative moment capacity. When the compact steel section is braced and the rebars are adequately developed in the negative moment regions, the nominal flexural strength may be determined from the plastic stress distribution shown.