From SSKE

Service is the application of specialized competences (knowledge and skills) for the benefit of another entity or the entity itself; hence, Service is a process of applying Resources. Operant resources participate always in service processes because they have the power to transform other resources (operant and operand) to produce effect.

Service can be categorised in two types: Direct Service and Indirect Service.

Service does not refer to (the physical attributes of) a process output, but it is a transcendental term for the activity of doing something for the interest or benefit of somebody else. Hence, service is a superordinate concept that includes both tangible and intangible goods (‘services’).Service is what is always exchanged, either in Direct Service interactions (services) or in Indirect Service interactions, through tangible goods (products). Self-service for instance, that is the application of resources by the beneficiary entity for itself, can be considered a special kind of Direct Service.