The short answer to that question is: all three, and a bit more!
The longer answer is that this is really for anyone who needs to have a professional relationship with anyone else. And professional relationships tend to fall into three categories:
Employees; that is, people who have to report to a boss.
Freelancers; that is, independent free-agents who work with a few different clients. There is a boss–and the boss is the client.
Partners, VP-s, or C-level executives at firms; that is, people who need to manage (often complex) relationships with other professionals. The partners are the boss!
Anyone who deals with complex relationships with customers. The book is not that useful for someone who just works at a cash register, smiles, and tells them the bill, and collects the money. And for relatively simple customer relationships, like perhaps working on the wait staff of a restaurant, the advice here is less useful. But when it gets to complex, long-term customer relationships, then the advice becomes powerful.
Finally, the advice here is a super-power for sales teams. While the book is not a sales book, in any complex sales process, the practical advice in the course is likely to supercharge the sales process and make the potential client want to work with you as much as your existing clients.