Fade is a gradient (yellow → pink → purple). “Fade percent” is a community scale: how much of the purple/pink is visible from the play-side. 80% still shows a large yellow tip; 100% is almost full purple-pink.
Most marketplace default sorts ignore %. A 100% Fade Karambit can be several times a 80% of the same wear. Always inspect seed / screenshot before paying a “max fade” ask.
On Dust2pedia the listed Fade price is the unfiltered floor. Filter CSFloat by fade % or pattern if you are buying a specific look.
Related skins
See also pattern index and overpay.