What are the SBML Levels?
Levels in SBML are a way of managing complexity in the continued evolution and enhancement of the language. SBML is being developed in a series of levels, where each level adds new features and fixes problems with the previous level. The lowest-numbered levels provide fundamental features that are common to all biochemical network models. Higher-numbered levels add more features that are specific to particular classes of tools. Any level can be used as a standard for interchanging models.