Singleton

Singleton classes are a bit of an oddity in the world of Operations Manager. Normal classes, if you may, are instantiated upon discovery. Simply defining a regular class and importing the management pack does not give you any objects to play around with, you have to discover them and connect their relationships when needed to be able to use them.
Singletons, however, are a completely different matter. A singleton do not require discovery. A singleton always exist and only exist once in a management group.

Now, what are they for? Most singletons you will deal with are the groups in Operations Manager. The moment you define a group of, say, computers or logical disks the group will exist, and there will never be a second instance of it. Ever. No discovery needed!
You will also run into singletons when building Distributed applications in your management pack.

One very important thing to notice about singletons is that they all exists on the Root Management Server. This is important because if you were to target a group, which is a singleton, with a monitor, that monitor will execute on the Root Management Server and not on the servers or instances within the group.