the X12 277RFAI-v6020 mapping for Line Items is:

Loop: 2220D Notes: SVC01 = id code qualifier, SVC02 is the claim amount, SVC04 is REV Code, Does not list line item

It "does not list line item" because if references the claim which has the actual line number.  Therefore is not direct a mapping from X12 to the CDex data element. 

Change mapping text to the following to clarify that this is an indirect mapping to the line number:

"Loop 2220D (Service Line Information) Segment SVC01 or SVC04 codes reference the claim service item, which contains the line item."