I don't think it describes density, though someone more informed than I might illuminate us. Any given (skeletal) muscle group (e.g. quadriceps, biceps, pectorals, etc) consists of both slow and fast twitch muscle fibers. Different muscle groups have different proportions of slow vs fast twitch, depending on the purpose of that group. For example, the average person's quads are have a roughly even distribution of slow vs fast twitch, but the muscles which we use to blink are almost entirely fast twitch.

There's a pretty good comparison table on Wikipedia if you're still curious, but once I see the ATP cycle coming up in a given article, I know I've reached the limits of my amateur understanding. Here, there be dragons.