At any floor location, only one thickness (depth) is used, and the object with the highest priority defines that thickness.

The thicknesses of overlapping objects do not add to define the thickness.

For example, you would expect the overall thickness of a drop panel located at a column to take priority over the slab thickness. By assigning a Priority to each object, the automatic mesh generator understands how to generate the finite elements.

The lowest Priority is 1. This is so that you can keep adding beams, thickenings and slab areas with higher priorities. There is no limit to the highest priority (other than your computer and text overflow).