Enhancements to the snapping behavior when working in shape edit mode allow you to ignore internal points and edges and use the Z value of elements as a snap point.
After entering into the SlabShape mode, click the Snap XYZ button in the Modify | Toposolid tab Modifier Elevation panel.
The value displayed near the cursor reports the elevation of the reference objects based on the Elevation Base setting. If Snap XYZ is selected and the mouse hovers over on a place that is not a reference object, the elevation follows the "Along surface" behavior.