Code 128 has three characters Set: A, B, C.

Code 128 has 106 different bar and space patterns and each pattern can have one of three different meanings, depending on which of three different character sets (characters Set A/B/ C) is employed.

Three different start characters tell the reader the character set used in the following encoding, and three shift codes permits changing character set inside the symbol.

So, sometime you can see some item like this: