B(0) is a measure of the extrapolated central surface brightness of
the disk component of the galaxy.  That is, the surface brightness
at zero radius.  Early measurments of galaxies that were easy to
detect suggested that B(0), which is physically a measure of the
surface mass density in the central regions of disk galaxies, was
a constant.  This would imply galaxy formation processes must target
some cloud of gas toward a fairly restrictive configuration.  Precisely
due to the discovery of Low Surface Brightness Galaxies, it is now
realized, but no yet accepted, that B(0) is probably not a constant
and that disk galaxies at many levels of central surface mass density
can be found.