The parity is like a backup or it take less space than normal backup with equal amount of data?
It's percentage based, but without a backup it's the best way to ensure integrity. I believe filesystem supported parity versions cannot do more than 100% parity per parity partition (I might be wrong).
Note: older/different versions of par2cmdline support more/less options, but I *think* the one at github has the patches applied for certain edge case collisions which older/other versions do have.