Though I hesitate to add another interpretation to the already long list of diagrams mapping the relationships between the nomenclature and specialties that fall under the umbrella of “user experience design”, this diagram has often helped me to explain my role and value proposition as an interface designer.
An appropriate balance between customer needs, business goals, and the opportunities and limitations afforded by technology provide an interface design's foundation in the form of a business case. A design that enables people to understand, use, and enjoy a product that fits the business case is what makes up the interface. All the other customer touch points (retail, support, marketing, etc.) form the complete user experience.
Over the past few years I've become more engaged in the development of product strategies in addition to product designs. In this role, the product design work is higher-level and the focus is on the transition from business model to user experience (of which an interface design is a key component).