UNFCCC Definition:

Completeness means that an inventory covers all sources and sinks as well as all gases included in the Revised 1996 IPCC Guidelines in addition to other existing relevant source/sink categories which are specific to individual Parties (and therefore may not be included in the Revised 1996 IPCC Guidelines). Completeness also means full geographic coverage of sources and sinks of a Party.

Examples: Party X does not provide estimates of HFC emissions from air conditioning, refrigeration, and other equipment. Party X notes that it does not have adequate data to estimate these emissions.

Because these emissions are unlikely to be insignificant, the submission from this Party is not complete.
Party Y has failed to submit a National Inventory Report (NIR) that includes information for all years of its time series. This failure is an even more significant lack of completeness.