The percentage is damage resistance (DR) and the flat number is damage threshold (DT).
When you take damage, the armor will either reduce it by the amount specified by the damage resistance or by the flat amount specified by the threshold, whichever is greater (not both).
So more DT is better against faster attacks that deal less damage (such as SMGs, knife attacks, etc), while more DR is better against heavy attacks (snipers, sledgehammers, etc).
In your particular case there's another modifier in the equation: tactical vests have their DT increased by 200% (tripled) against firearms. So they are hard to penetrate with firearms, especially when augmented with anti-rifle plates. On the flip side, they have lower DR and generally lower base DT (when not multiplied) so they are not as good versus other types of attack.
Hope this clears it up.