What controls the depth where the tabs are in a toolpath?

The bottom of tabs are always at the Cut Depth for the toolpath and extend upward by the Tab Thickness set during the tab setup.

If you set a Cut Depth deeper than the Material Thickness to intentionally cut past the bottom of the material, be sure the tab Thickness is enough to still appear in the material. 

Also, when using a Start Depth other then 0.0 with tabs, if the Start Depth is below the top of the tabs, some or all of the tabs will be machined away