what's the difference btw the different underground surface types in the createHBSrfs component?

May I ask the difference btw the three different types of underground surfaces, i.e. underground wall, ceiling and slab, in the createHBSrfs component?

1. Are they defined by the surface normal?

2. For an interior floor surface touching the ground, should it be specified as 2.25UndergroundSlab?

3. Do we need to specify the outside boundary condition for each individual surface before "assembling" them as a HBZone? Or can we leave the solve adjacency component to deal with it automatically later?