Just as stars can be intrinsically bright or intrinsically faint, galaxies of a particular Hubble classification can also exhibit a range of brightness. For this reason, Van den Bergh introduced an additional descriptor for spiral galaxies - a galaxy luminosity class.
In this system, each galaxy of a particular Hubble type is assigned a luminosity class ranging from I for the brightest, to V for the faintest galaxies. Since total optical light roughly corresponds to the total mass of visible stars, luminosity class I galaxies within a Hubble type are also the most massive and have the most prominent spiral arms.
The luminosity class for galaxies also correlates well with the regularity of spiral structure, with the most luminous (most massive) galaxies being the most ordered. This is quite simply explained by the density wave model of spiral arm formation. The more massive the galaxy, more gas clouds will concentrate in the regions of high density. Since more gas is available, more new stars will be formed at the site making the spiral structure better defined.