As used by the IPCC, this term denotes "a coherent, internally consistent and plausible description of a possible future state of the world."
1. They are not forecasts. Each scenario is an alternative image of how the future can unfold. A projection can serve as the raw material for a scenario, but scenarios often require additional information (e.g., about baseline conditions).
2. A set of scenarios is often adopted to reflect, as well as possible, the range of uncertainty in projections.