When body armor is hit with a bullet it responds with nearly the same force thus that it will not be penetrated. Soft body armor and Hard plate carriers function in different ways but serve the same crucial function.
Soft Body Armor is made out of tightly woven fabricand has been traditionally a solution of every day use that is easily concealed under clothing. A necessity in cases where wearing large bulky metal body armor may be cumbersome.
Hard body armor has become increasingly light with the use of alternative materials. These days many new types of plate carriers are made out of ceramic and can be combined with hard plastics like polyethylene.
RMA Armament has created a plate that disperses the kinetic energy of the bullet as soon as it makes contact. When the bullet hits, the ceramic plate breaks the round instantly and causes it to disintegrate. A polyethylene backing absorbs the kinetic energy of the fired round and thus protecting your vital organs. (How Stuff Works)
How to size your your plate carrier:
Front plateTop of front plate is at the jugular notch, bottom of the plate is 4-5 inches above your belt line or right at your belly button.
Back plate As a general rule the top edge of the back plate should align with the top edge of the front plate when viewed from the side. The bottom edge of the back plate should be 2 inches above the belt line.
Plate Length Placing the plate at your belly button and above you belt line prevents the plate from pushing upwards into the neck area as you move into different positions running walking or standing.
The soul purpose of body armor is to cover the vital organs such as heart, lungs and main veins. Remember that there are still many other exposed areas of your body. You must pair the use of body armor with proper use of your firearms & techniques for moving, combat and finding cover.