When you create a new drilling operation, the default heights for Top Height and Bottom Height, are Hole top and Hole bottom, respectively. With this default, the entire depth of the selected face(s) is machined - and if Auto-merge hole segments is enabled, the depth of the entire hole is machined.
In some cases, you may need to drill only a part of the depth of the hole. As an example, you can select Hole top for the Bottom Height, in effect giving you the option to drill, say, 1 mm down from the top of every hole.
The following images show two possible combinations of the drill height settings.
Bottom Height "Hole bottom". Auto-merge hole segments is enabled
Bottom Height "Hole top", with -1 mm offset applied. This setting drills down 1 mm from the top edge of the hole; effective if you are spot drilling a selection of holes.
Spot drilling multiple hole sizes and depths in one operation