If you mean selecting all elements under a toposurface, this is difficult since some families are "Level" related and others (like genrei models) don't have this indication. My guess is you need to get the location of your slope and loop over all elements, ask there boundigbox and check if the coordinates are below the slope.
I have a wall on a sloped surface while taking area and volume it gives area and volume of the whole wall. What I want is that it should give me the area and volume of the wall above slope excluding below slope.
hmm, as far as I know there isn't a "standard" API call to do this, Somehow you need to get the points (contour) of the sloped surface, project the points to the wall and do the math... Not an easy task.. I'm afraid I can't help you on this, sorry.
