ICD-10-CM far exceeds its predecessors in the number of concepts and 
codes provided. The disease classification has been expanded to 
include health-related conditions and to provide greater specificity 
at the sixth digit level and with a seventh digit extension. The 
sixth and seventh characters are not optional; they are intended for 
use in recording the information documented in the clinical record.

http://www.cdc.gov/nchs/about/otheract/icd9/icd10cm.htm

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