The workspace is the place in your disk used to work against your SCM. But, what is loaded into a workspace?
You decide what you have on your workspace depending on the way your team wants to work. And the way to tell Plastic what you want to see is by setting a selector. Each workspace has an associated selector, indicating the rules that load workspace contents.
Selectors are used to specify what branch, changeset, marker or checkout rule to apply for different items in your workspace, or even to mount several repositories on a single workspace.

As soon as you create a workspace and start working, the server will take care of what you have placed on it. You could, for instance, move a workspace from one machine to another, without having to move the physical files, just telling the Plastic server that the workspace is now on a new location.