Eric's analogy about interface and application (see comments for the previous post, Business services versus capabilities) maybe of wider interest and hence, this post.
The capability construct can be illustrated with ease in 3D on the EA FFLV framework representation (see picture), as a View that delivers a service, View made of the Functions, Flows, and the resource Layers, (people and technology) executing them.
The FFLV framework consists of Functions, Flows, Layers and Views as illustrated. The capability is a View.
The Help Desk capability, the Printing capability... and non-it capabilities such as On-line Sales... can be all be represented as Views in this framework while still integrated in the EA whole.
The business services can be easily illustrated too since they are formalised capabilities, with transparent execution for its customers. The framework metamodel would exhibit the elements to further describe a business service.
In fact, in any enterprise there is a mixture of them: some are capabilities while some are already business services such as Cloud or Web services).
In the end, an Enterprise and its architecture can be illustrated as the sum of its capabilities and the EA as the sum of the capability Views.
You have to see FFLV at work first. Then it all comes simple. No fuzziness.
The "An Enterprise Architecture development framework" book on Amazon would help.