There isn't an objective density at any of the zones, at least not across all film/developer/paper combos. The Zone System presents a conceptual/subjective framework to allow pre-visualization of a scene to anticipate the best way to expose/develop/print. II-VIII are called 'textural' for the simple, direct reason that any textures in the print are visible in those zones. I is dead black, no texture. IX is spectral highlights, no texture. (Although, I work from III-VII, and bracket if I'm in doubt). The Zone system(s) are nowhere near as complicated as sometimes made out. You really should read Ansel Adams' books. Old copies are quite inexpensive. They are classics of photographic literature and great reads, if nothing else.