The IETF has produced a considerable number of data models in the YANG modelling language. The majority of these are used to model devices and they allow access for configuration and to read operational status. A small number of YANG models are used to model services (for example, the Layer Three Virtual Private Network Service Model produced by the L3SM working group). This document briefly sets out the scope of and purpose of an IETF service model, and it shows where a service model might fit into a Software Defined Networking architecture or deployment.