Surface brightness
Also called: SB, mag/arcmin².
How bright an object is per patch of sky — what really decides whether a galaxy or nebula is easy to see.
Two galaxies of the same total magnitude can be easy or nearly invisible: if one packs its light into a small disk and the other spreads it over a big area, the big one has a low surface brightness and barely stands out from the sky. That's why M 33 and M 101, bright on paper, are notoriously hard from light-polluted places.
It's given in magnitudes per square arcminute (or arcsecond); smaller numbers are brighter. Magnification doesn't change it — but it darkens the sky background, which helps small objects.
In AstroPilot
AstroPilot's visibility ratings use surface brightness, not just total magnitude.