1. The Base Model Layer (BML): At the final development state, this layer comprises only a central time integration management and a run control facility for the processes involved. In the transition state (at present) the BML is the domain specific model with all modularized parts removed. For instance, in case of an atmospheric model it is usually a GCM.
2. The Base Model Interface Layer (BMIL), which comprises basically three functionalities:
   • The central submodelmanagement interface allows the base modelto control (i.e. to switch and call) the submodels.
   • The data transfer/export interface organizes the data transfer between the submodelsand the base modeland between different submodels. It is furthermore responsible for the output of results (export). Based on the requirements of the model setup, the data can be classified according to their use, e.g., as physical constants, as time varying physical fields, and as tracers (i.e., chemical compounds).
   • The data import interface is used for flexible (i.e., grid independent) import of gridded initial and time dependent boundary conditions.
   The BMIL therefore comprises the whole MESSy infrastructure which is organized in so called generic submodels(see Appendix A).
3. The Submodel Interface Layer (SMIL): This layer is a submodel-specific interface, which collects all relevant information/data from the BMIL, transfers them via parameter lists to the Submodel Core Layer (SMCL, see below), calls the SMCL routines, and distributes the calculated results from the parameter lists back to the BMIL. Since this layer performs the data exchange for the submodel, also the coupling/feedback between different submodelsis managed within this layer.
4. The Submodel Core Layer (SMCL): This layer comprises the self-consistent core routines of a submodel (e.g., chemical integrations, physics, parameterizations, diagnostic calculations, etc.), which are completely independent of the implementation of the base model. Information exchange is solely performed via parameter lists of the subroutines. The output is completely determined by the input.

Further details can be found in:
Patrick Jöckel, Rolf Sander, Astrid Kerkweg, Holger Tost, and Jos Lelieveld, Technical Note: The Modular Earth Submodel System (MESSy) - a new approach towards Earth System Modeling, Atmos. Chem. Phys., 5, 433-444, SRef-ID: 1680-7324/acp/2005-5-433, 2005. http://www.atmos-chem-phys.net/5/433/2005