identification of the gun that fired a bullet from an analysis of the unique marks that every gun makes on the bullet it fires and on the shell ejected from it
evidence proving that you are who you say you are; evidence establishing that you are among the group of people already known to the system; recognition by the system leads to acceptance
evidence proving that you are not who you say you are not; evidence establishing that you are not among a group of people already known to the system; recognition by the system leads to rejection