Services in DiF are part of a Hierarchical Model that is defined in the following entities:
Providers are companies of institutions that supply packs of services to a system. I.e: Digitalis, Google, SUN, etc.
Applications is a pretty straight forward concept. A collection of services come together in the form of applications. Each provider can supply several applications. I.e.: Online shop, HR Suite, etc.
A piece of an application. A specific functionality that we wish implemented. I.e: Manage users, Make a purchase, etc.
A specific task. Part of a service. All services can be decomposed in one or more stages, thus dividing the logic of the execution steps of the service functionality in these unitary stages.
The Stage entity acts as the controller on the traditional MVC model. As such, it should contain the business logic (application behavior) and might present view redirecting capabilities based on the application state. So, a Stage can have zero (you can have your stage redirecting the execution flow to another stage!), one or many views associated. As for the "M" in the MVC pattern, the model, check out the Model section.
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See an example in the following tutorial.