I have not seen an answer to this basic question that eludes me.
The volume option property of the feature definition that is assigned to the 3D meshes will determine how they are reported for earthworks. There is an outline of the volume options here: https://communities.bentley.com/products/road___site_design/w/road_and_site_design__wiki/42267/what-do-each-of-the-volume-options-mean-for-the-mesh-property-feature-definition