Badges, also known as Skills, Skill Badges are equip that usually not damage zombie and can't be control manually. Instead, it provide modifier in all category. From damage to ammo to HP to supply. They can be worn 3 pieces, usually be able to worn same piece more than once, but some limit to only one.
Because modify they create, it can flip whole game if choose wisely.
Early badge focus on keeping player alive.
Mid-game badge provide dodge and other useful ability.
Late-game badge provide damage modifier. Some can cause one hit kill against boss.