This paper presents a Usable developer-oriented Functionality Composition Language (UFCL) designed for ubiquitous systems developers. Easy to write, this language is used to semantically describe functionalities implemented by services in a service oriented architecture where each service exposes its own description. Service factories can also be described using UFCL: a factory defines an abstract composition pattern and is able to instantiate product services on demand. This paper also describes UFCL compilation that makes it possible to reason about functionalities exposed by services and factories.