Directly from Crutchfield:
Top-mount vs. bottom-mount
Second, will you top-mount or bottom-mount your speakers? In a top-mount, the lip of the speaker rests over the edge of the hole. This method usually requires less mounting depth and a little less labor. A bottom-mount involves recessing the entire speaker into the hole so that no part protrudes — perfect if you need to create clean lines. If you plan on mounting your tweeter on the woofer, however, the tweeter may stick out slightly from the woofer plane. Since you will need to fit the grille over the speaker, make sure the speaker protrusion does not exceed the grille depth.
As you can see, Top Mounting Depth comes into play if you are inserting the speaker THRU the hole in the panel, from the outside of the panel - hence the speaker is 'Top Mounted' on the panel.
Bottom Mounting Depth is the critical dimension we need to reference. As our speakers install from the rear of the panel, interior to door with the face of the speaker flush against the inside surface of the door panel (or adapter) - or Bottom Mounted on the panel