Box mounting depth question

[DjDan101]

On a box for my truck that I'm looking at, it says the mounting depth for a 10 is 6.5 inches. A sub that I found that I want is the same number, and the airspace almost matches up. Does that mean that there is only 6.5 inches of clearance from the hole in the box to the bottom of it, or that is the most that should be mounted in it and it still be optimal? Here is a link to the box just in case you need to see it https://www.soundoffaudio.com/item.cfm/id/732

[DjDan101]

Bump