The SelectPointsAboveOrBelowSurface command allows the user to create a selection of points lying either Above or Below a chosen Surface.
Type SelectPointsAboveOrBelowSurface at the command line. You will be presented with a form to choose which Surface to use, which method - Above or Below - to use, and to optionally limit the selection to points in one or more Point Groups. Make the desired selections, press OK, and the corresponding Points will become selected. You can choose to have your selections saved for the next use of this command.
As command line alternatives, there are the 2 commands SelectPointsAboveSurface and SelectPointsBelowSurface which will operate on all points and just present you with a small form with which to choose the Surface.
Note that you may not see the selection on screen as gripped objects, as the number of gripped objects is limited by the AutoCAD Sysvar GRIPOBJLIMIT.