And just to make a few more connections from Thilo's explanation and put it in terms of "stops":
A luminance range of 8:1 or 8x is equivalent to 3 stops. If you had a filter with an exposure factor of 8x, that would also be three stops. In log terms, an ND filter with a density of .3 is 1 stop, and a ND filter with a density of .9 is also three stops--the same three stops as the density range of Thilo's hypothetical image, which had a density range of 0.9 logD, the same units as are used for describing neutral density filters.
This is also the same unit used on a densitometer, so when Ansel Adams says in the context of the discussion of the development test that he looks for a Zone VIII density of 1.2 for printing on a diffusion enlarger, that means 4 stops above the base+fog density. A film with a Dmax of 2.7 would have a maximum range of 9 stops.