Some time ago I was listening to Sol Ahimsa speaking about densities, and she was talking about the direction of dimensions- densities were on a 90 degree angle to the dimensions, which confused me, but it seemed to make sense (planes and dimensions are measurements not things), 4-Dimension being time, yet she was explaining that each plane has a number of densities, which confuses the issue when you use D as your initial- you can have 2 Densities in the Astral and a 3 or 4 Densities in the RTZ and 4 densities in the physical, and no density at all in the mental plane.
I'm just trying to illustrate how interesting- and often arbitrary this stuff is.
I'm happier with planes as environments with psychological distance as a measuring rod and leave it at that.