You can snap to reference objects to adjust the slab-shaped elements such as Floor, Roof, Toposolid, etc.
After entering into the Slab Shape modify mode, toggle the checkbox to enable and disable the snap ability on the inner points and lines:
Modify | Toposolid tab 
Shape Editing panel Snap to Editor Points and Lines
To snap on XY of the reference object, in the Modify tab Modifier Elevation panel, choose
Along Surface
or
Absolute
:
- Along Surface
- Points are added in relation to the surface. Value displayed near the cursor reports the elevation of the surface based on the Elevation Base setting. Points will be added using the Offset from Surface value.
- Absolute
- Points are added based on the Elevation Value. Value displayed near the cursor reports the elevation of the surface based on the Elevation Base setting.
Choose Snap XYZ to snap on the XY and Z of the reference object. Value displayed near the cursor reports the elevation of the reference objects based on the Elevation Base setting.
Use Offset from Snaps to indicate the Z value offset. The point will be placed at the elevation = Refferal Z + Offset Z.
Remote objects should be turned off if you use Slab Shape editor for large models.
See also Add a Point for Shape Editing