Damage from rifle rounds and the armor piercing FN57 ammunition (used in the Five-seveN and P90) is reduced only by about 25%, although it should be noted that the FN57 ammunition does little damage in the first place. As long as you have any armor at all, the exact armor value does not matter. It should also be noted that Kevlar will not protect you from attacks landed anywhere other than on the chest, stomach, and head (with a helmet) hitboxes.
There is no visual difference when having armor equipped thus making it easy to fool other players. Enemies can only tell if a target has armor by damaging them and seeing white sparks which indicates that the player is wearing armor.
If a mapper has decided to use content from Half-Life and place the battery used specifically for the H.E.V. suit, players will gain 15 points of armor. In some custom servers, if a Half-Life NPC is used, the Kevlar has the same properties as the H.E.V. suit.
Although the world model of the Kevlar does not appear in official maps, some custom maps feature the MDL file that can be obtained by players.