Tier 1b in the IPCC Good Practice Guidance includes updated equations from the Revised 1996 IPCC Guidelines (Tier 1a). The selection of a Tier 1a or Tier 1b method is not related to whether the category is a key category but to the availability of activity data and differentiated EF for each nitrogen source.

Preference is to be given to the Tier 1b equations in the IPCC Good Practice Guidance, which expand the number of terms in the equations. However, for Parties that do not have the necessary data for Tier 1b, the use of Tier 1a equations is acceptable.