These are the people who build, operate, maintain, and repair the stuff that the experts design and theorize about. Theirs is a highly technical knowledge as well, but of a more practical nature. These are the people who make business, economic, administrative, legal, governmental, political decisions on the stuff that the experts and technicians work with. If it's a new product, they decide whether to produce and market it. If it's a new power technology, they decide whether the city should implement it.
Executives are likely to have as little technical knowledge about the subject as nonspecialists.
Their interest may be as practical as technicians', but in a different way. It's important to determine which of the four categories just discussed the potential readers of your document belong to, but that's not the end of it. One of your most important concerns is just how much knowledge, experience, or training you can expect in your readers. If you say no, then you run the risk of customers' getting frustrated with your product. More than one audience.