Isn't a Mental Model Something Else, Not a Diagram?
Frequently when people hear the phrase "mental model" they think of a narrow slice of Don Norman's definition. In his book The Design of Everyday Things, right after discussing his example of the refrigerator/freezer controls, Don Norman writes this definition of mental models: "the models people have of themselves, others, the environment, and the things with which they interact." (page 17 in the First Doubleday/Currency Edition, 1990) Because many of us are interaction or information designers, we mostly focus on the "models people have of things with which they interact" part of the definition. It's good to remember that Don's definition is broader.
Mental model diagrams are your models of "others." Not only that, these mental models are not necessarily just your model, but your whole design and development team's model. I would like to define that term "others" more specifically, too. "Others" is a particular audience segment (or persona) trying to do a particular thing. For example, it is people who are "Passionate About a Topic" choosing which college to attend, or people who are "Exploring Paths" choosing a major to study. It is "evaluators" deciding which data management environment to invest in for their business. It is people who "Think It Through" considering how to meet people they might be interested in dating. It is people "Uncomfortable with The System" coping with a health problem.
Keep in mind you might want to model your internal employees and service providers as well. A hotel might want to model what front desk and concierge folks do. In addition to lunchtime patrons who want to "Shake It Up," a fast food chain might study the employees making the burgers during the lunch rush. Realize you can model a wide range of groups. Your team can create several shared models that help you support and design for all sorts of various "others."
And yes, the top half of these mental model diagrams is really the mental model part; the bottom half is where you align the support your organization offers people, whether they are internal or external. By matching services, processes, and information under each tower, you can see if they fit what people are trying to accomplish. You will see where there are strong and weak matches, or no matches at all.
When I first proposed the book title, the board of directors at Rosenfeld Media felt that a new term was needed. They suggested "alignment diagrams." Early clients and readers convinced them otherwise.