"A source or supply from which benefit is produced, typically a limited amount" (Wikipedia). The Resource category groups all the concepts corresponding to what is sold. Often the resource to be sold is an asset which is defined by Wikipedia as any resource owned by a business.
Most of the time, the resource is a physical object. However, often, the resource to be sold is a physical device not yet built or a service. In that case, we define the resource by a set of requirements.
The gist:Accounting ontology already defines it for you as: "a documented physical or functional need that a particular design, product, or process must be able to perform. Alternately, the obligation of a person or organization to behave in a certain way (i.e., drive on the right side of the road)".
The resource is sometimes also about the person or an organization who is temporary going to work for another company. A typical example is a consultant.