Depth 0 is the top of the drawing order, with increasing depths being underneath objects with smaller depth. The maximum depth is 999.View menu.
Clicking on the checkbox area of a depth indicator will turn on or off the display and editing of objects at that depth. By clicking on one checkbox and holding down the mouse button while dragging the mouse up or down over other depth checkboxes, multiple depths may be enabled or disabled at one time.
If you click on a checkbox that is ON and drag the mouse over other checkboxes, they will be turned OFF regardless of their initial state. Conversely, if you click on a checkbox that is OFF and drag the mouse over other checkboxes, they will be turned ON.
When printing/exporting, it is possible to choose to output only the objects with the depths set ON here (active layers), or output all objects (all layers) regardless to this setting.