The principle of CKD records is that since data block lengths can vary, each block has an associated count field which identifies the block and indicates the size of the key, if used (user-defined up to 255 bytes), and the size of the data area, if used. The count field has the identification of the record in cylinder-head-record format, the length of the key, and the length of the data. The key may be omitted or consist of a string of characters.