HYBL

I downloaded one of the RUC2 grids from ftpprd.ncep.noaa.gov
and passed it through dcgrib.  The grid is:

ruc2.t23z.bgrbf01

I looked at the resulting GEMPAK file and the vertical coordinate
is labeled HYBL, and ranges from 40 to 1.  Does anyone know
what that is, and what units?

Thanks,
Robert Mullenax