When the design process starts with human requirements we realize that functionality is all about the human interface. User needs determine the form from the inside-out. The maxim "beauty is more than skin deep" is as true for every artifact of design as it is for people.
A well-designed automobile, for example, responds to ergonomic requirements. The seat should have good lumbar support and adjust to many body types, the instrument panel should be read and understood at a glance, the pedals should provide proper resistance and feedback, visibility should give the driver a good view of what's ahead, beside and behind the car, and so forth. But the automobile's user interface does not begin and end with the driver operating the car or the passengers riding in comfort. What happens when that automobile encounters another one at a combined speed of 60 to 120 miles per hour, for example? As the body of the car changes shape in a rather radical fashion, we have a completely different kind of user interface design problem.
For every product there is a host of users that also have interface design requirements. These users are usually forgotten in the design process. Their interface problems entail the manufacture, distribution and maintenance of the product. The owner of the product may himself perform some of that maintenance and repairs. And whereas most products end up as garbage (save the few preserved as antiques), there is a disposal and recycling human interface problem.
Consider how a given product interfaces to human well-being as it performs as an element in a complex system composed of many like products. The automobile, for example, as an element in a complex transportation system manifests many more complex user interface problems: the social impacts on lifestyles, environmental impacts on the shape of cities, suburbs and the countryside, not to mention pollutants. Similarly, with the rise of the internet the individual computer is an element in a complex communications system which raises issues of privacy and security.
Each of these problem areas are part of the user interface design paradigm. To the extent that humans are impacted by and interact with the object of design, there are many stakeholders that are involuntary "users".
Joint projects with other designer companies and consultants are welcome.