There is a strong case that this sequence of Domains [from 1 to 4] is the order in which the Carbon Budget Accounting in CBAT should be analysed if it is to be policy-relevant.
The objective of the UNFCCC is achieving stable and safe GHG concentrations in the global atmosphere. Policy, if it is to be relevant at all, is fundamentally governed by that objective and the need to achieve it - in a phrase UNFCCC-compliance.
So CBAT Domain One is first and foremost an enquiry into the relationship between future contraction of global carbon emissions - both potential budget and potential feedback emissions - and the global atmospheric concentrations that result from these.
In other words, prior to considering the: -
Ideological/constitutional issues arising in Domain Two
Technological/fiscal issues arising in Domain Three
Cost/Benefit issues arising in Domain Four
CBAT Domain One lays out a logical trend-investigation/quantification of the realtionship between emissions and concentrations. In principle this is a simple and familiar stock/flow model as here
A more detailed look at the scientific questions related to Contraction and Concentrations relationship and UNFCCC-compliance is laid out here