If the mass excess is negative, the nucleus has more binding energy than 12C, and vice versa.
If a nucleus has a large excess of mass compared to a nearby nuclear species, it can radioactively decay, releasing energy.
Nuclear reaction kinematics are customarily performed in units involving the electronvolt, which derives from accelerator technology.
The combination of this practical point with the theoretical relation E = mc2 makes the unit megaelectronvolt over the speed of light squared (MeV/c2) a convenient form in which to express nuclear mass.
The mass excess can be converted into energy using 1 Da = 931.494 MeV/c2, and E = mc2, yielding 166.737 MeV.