The speaker's mounting depth when it's installed as a bottom-mount. Your vehicle will need this much clearance in order for the speaker to fit properly. This is an important dimension to consider if you're installing your new speakers in a non-factory speaker location. If you're installing a subwoofer in a box, you'll need to make sure there is this much clearance between the inside of the box opening and the opposing inside surface.
If you're installing your new speakers in an existing factory speaker location, we've taken out the guesswork for you! Just use the Crutchfield Vehicle Selector to see only the speakers that'll fit in your vehicle.