What is a level 5 in taping??? (drywall)?

Level 4 is seams taped and coated with feathered-out coats of mud, fasteners filled twice. Level 5 takes it a step further by skim-coating the entire surface with a thin layer of mud to mask the difference in texture that exists between drywall paper and sanded compound. Some new very thick primers claim to provide a level 5 equivalent look, but i have never tried them. The layer of mud to be applied is really very thin, just enough to cover the texture of the paper, it has a porosity to it that shows when painted. This is only really necessary for very picky homeowners or for darker glossy paints.