This diagram shows that the Singleton pattern consists of a single class.
This class takes the role of the Singleton in the operation. However, it need not be called Singleton. In general it will not be.
This class has a single field called theInstance which points to the object that is the unique instance.
The applicability of a pattern describes the circumstances under which the pattern is appropriate; that is, when you should and should not use it.
The Singleton class has two methods, a static getInstance() method that returns a pointer or reference to the unique instance and a protected constructor.