Using the LISP filter "-4" you can easily select DWG objects, which have their definition/reference point above or below a given limit set on the individual X,Y,Z components of their coordinates. So you can select/filter objects with a reference point in a positive or negative Z-elevation, or in the positive XY half-space, or in the positive XYZ octant.
See examples of selecttion sets (can be input on the "Select objects:" prompt)
Negative Z-elevation:
(ssget "_X" '((-4 . "*,*,<")(10 0 0 0)))
Positive XY half-space, any Z:
(ssget "_X" '((-4 . ">=,>=,*")(10 0 0 0)))
Positive XY half-space, only "zero" Z (flat):
(ssget "_X" '((-4 . ">=,>=,=")(10 0 0 0)))
Positive octant XYZ:
(ssget "_X" '((-4 . ">=,>=,>=")(10 0 0 0)))
You can also combine the filters through AND, OR, NOT. Please note that only the reference point is checked (e.g. a circle center, polyline starting point), not the whole bounding box of a drawing entity.