Quoted from: https://www.pbl.nl/en/publications/FAIR_1_0__Framework_to_Assess_International_Regimes_for_differentiation_of_commitments 

Over the years, many approaches for burden sharing / differentiation of (future) commitments in international climate policy making have been proposed, both in academic and policy circles. The FAlR model presented in this report does not intend to promote any particular approach to international burden sharing. Instead it aims at providing some menu for choice to support policy makers in evaluating options for differentiation of (future) commitments (den Elzen et al., 1999). FAIR is a decision-support tool (a so-called 'scanner'), which allows the user to interactively evaluate the implications of different approaches and criteria for international burden sharing, in terms of the allocations of allowed emissions (emission permits). Allowed emissions are calculated on a regional basis for 2 (Annex I and non-Annex l), 4 (OECD Annex I, non-OECD Annex I, ASia and other developing regions), or 13 world reg1ons, as well as a number of selected countries. By including a simple climate model, called meta-IMAGE 2.1 (den Elzen，1998), based on the integrated climate change assessment model IMAGE 2.1 (Alcamo et al., 1998), the FAIR model enables users to relate burden sharing schemes to global climate protection targets, such as global mean surface temperature change and sea-level rise.

Four basic modes of FAIR

FAIR consists of four basic model modes:

l. Scenario construction: in this mode the climate impacts of pre-defined or own-constructed global emissions profiles for greenhouse gases can be scanned.

2. Increasing participation: in this mode the number of parties involved and their level of commitment gradually increase according to participation and differentiation rules, such as per-capita income, per capita emissions, or contribution to global warming.

3. Convergence: in this mode all parties participate in the burden-sharing regime, with emission rights converging to equal per capita levels over time.

4. Triptych: different burden sharing rules are applied for different sectors (e.g.convergence of per capita emissions in the domestic sector, efficiency and de-carbonisation targets for the industry sector and the power generation sector).