UNFCCC Definition:

Comparability means that estimates of emissions and removals reported by Parties in inventories should be comparable among Parties. For this purpose, Parties should use the methodologies and formats agreed by the Conference of the Parties (COP) for estimating and reporting inventories.

The allocation of different source/sink categories should follow the split of the Revised 1996 IPCC Guidelines, at the level of its summary and sectoral tables.

Example: Party X has defined all airline flights by foreign-flagged airlines as international bunkers, while all flights made by their domestic-flagged airlines as domestic aviation (Civil aviation category). The Party collects data from each airline, but this data does not distinguish between international and domestic flight segments.

Because this definition of aviation international bunker fuels is different than the IPCC’s definition, Party X’s estimates of international aviation bunker fuel emissions are not comparable with those of other Parties.