I will try to answer that.
The A1, A2, B1, B2 and the like do not refer to models. They refer to IPCC emission scenarios. Some have low emissions. Some have much larger. But the modellers, the Francis Zwiers and Andrew Weavers of the world, will then take those scenarios that are projections—there is nothing absolute in them, but they're all plausible—and go from the emissions to the concentrations in the atmosphere and then insert those concentrations into their models to see what the models project for that sort of scenario.