From the description of the Model class in the
MVVM chapter of the Prism MSDN documentation:
"The model in the MVVM pattern encapsulates business logic and data. Business logic is defined as any application logic that is concerned with the retrieval and management of application data and for making sure that any business rules that ensure data
consistency and validity are imposed. To maximize re-use opportunities, models should not contain any use case–specific or user task–specific behavior or application logic."
Therefore, if you need to expose certain functionality or use case that would be consumed from other components under a certain interface, you could encapsulate it into a service. Prism provides the means to share that service (you can read more about it
in the Shared Services section inside the
Communication chapter of the Prism MSDN documentation) across different modules. The decision on whether to use a service or not depends on your requirements and preference for the architecture of your application.
I hope you find this helpful.
Guido Leandro Maliandi