,,,does that mean that your measurement should be taken from floor to the bottom of your box ,,or the center of your box???
As has already been said this should be spelled out somewhere on the prints or in the specs.
Most of the prints we get use the center of the box, however for fire alarm devices they give us the height to top of the device which is different from the height to the top of the box.
Do not forget that AFF includes floor finish too, so if you are roughing in do not forget to add the height of the floor finish.