The interface between business viability and design desirability is behaviors.

The success of every business initiative hinges on customer behaviors, and to a slightly lesser extent the behaviors of people on the front lines who serve those customers, and the behaviors of many others who support the front line.

Design specializes in what motivates those behaviors, which inhibits or redirects them, and what kinds of conditions encourage networks of behavior that produce favorable business outcomes.

The interface between operational feasibility and design “desirability” (I prefer “adoptable” or “joinable” or even “partakable”) is the shaping of the actual conditions that produce the behaviors. This, of course involves the production of the customer touchpoints designers shape. But in service design, internal touchpoints exist at every interface between people and the rest of the organization. Besides touchpoints, there are also organizational infrastructure that are not touchpoints in any conventional sense — policies, team structures, technologies, partnerships, etc. These enable or constrain conditions for behaviors, and these are carefully considered and, where appropriate, changed.

The interface between business viability and operational feasibility is the initial and ongoing investment to produce the conditions that will produce the behavioral return on investment.

The overlap of all three together is what I call the field of service system development, one third of which is service design, fluent in business viability and operational feasibility, but specializing in adoption/participation factors.