What is the usage for Dog Tag ?

During the American civil war, the "dog" appears in the U.S. military, war inevitable blood, if after his sacrifice, a "dog" brand, other soldiers can find themselves, in the "dog" brand has its own name, rank, their own paper or wood is done before, but are often destroyed by war, so people will directly put the names on the coin.During the second world war, the content of "dog tag" was also being improved. On the battlefield, there was smoke everywhere, and no one knew anyone. Some people needed to be treated.Today, the material of "dog" brand into the stainless steel, and have two brands, the size of all round with black collar, neck chains have also changed, now can have above the "dog" brand name, service number, type, and even the information such as address, religious beliefs, information is more and more perfect, better find themselves in others.In terms of material, there may be more changes in the future !