The check digit is used to confirm the initial digits of the card number. The benefits of using a checksum is that it prevents casual attempts to invent credit card numbers, as only one in ten will be valid.
Length (adj). the longest, or longer, dimension of any object, in distinction from breadth or width; extent of anything from end to end; the longest line which can be drawn through a body, parallel to its sides; as, the length of a church, or of a ship; the length of a rope or line.
1. the linear extent or measurement of something from end to end, usually being the longest dimension or, for something fixed, the longest horizontal dimension. 2. the extent of something from beginning to end, measured in some more or less regular units or intervals: the book was 600 pages in length.