What is a blood type chart used for?

Blood charts are used to typically identify what blood type an individual has. The test is simple one; there's an anti-serum used to type the blood. If it clumps with Anti-A serum and not the B serum, you are a type A; if you clump with Anti-B serum and not A serum you are type B, if it clumps to Anti-A and B serums, then you are an AB, if doesn't clump to either Anti-A or B serum you are a type O.