CCSM coupled model is based on a framework which divides the complete climate system into component models connected by a coupler. This design requires four component models - atmosphere, sea-ice, land, and ocean - each connected to the coupler, and each exchanging data with the coupler only.
The Climatological Data Atmosphere Model (datm) functions as the atmosphere component in a CCSM configuration. The datm atmosphere component interacts with the coupler just like any atmosphere model would, but it is not an active model, rather, it takes atmosphere data from an input data file and sends it to the coupler, ignoring any forcing data received from the coupler. Typically the input data file contains mean daily data generated by an active CCSM atmosphere model (it's unlikely that real world observations exists for all the required fields). Such a "dummy" atmosphere model is useful for doing ocean + ice spinup runs.
Important note: When assembling a CCSM configuration, the user must carefully consider the limitations and requirements of all components and make sure that the complete set of component models will interact in a meaningful way. In particular, the user must verify that the data provided by this model is adequate for their specific application.