Layperson’s Definition:

These are the big source or sink categories and the ones that have atypical (i.e., differ from the average) emission trends that could significantly impact total emissions if there were problems with their estimation.

IPCC Definition:

According to the IPCC Good Practice Guidance, a key source or sink category (key category) is one that is prioritised within the national inventory system because its estimate has a significant influence on a country’s total inventory of direct greenhouse gases in terms of the absolute level of emissions, the trend in emissions, or both.

The methods for determining Key Categories are discussed in Chapter 7 “Methodological Choice and Recalculation” of the IPCC Good Practice Guidance and in Chapter 5.4 of the IPCC Good Practice Guidance for LULUCF.

As a reviewer, the identification of key categories provides you a useful tool for prioritizing your work by helping you concentrate on the most “important” categories in a Party’s inventory.