Most controls allow you to specify parameters to customize the control. Currently, most controls support the read-only parameter, which is very useful when you want to build a task for displaying information, without allowing the user to modify it. Choosing to make a field read-only both disables the user interface and gives the users read access to the attributes (if you are using Role-Based Services).
The values parameter is useful for limiting the possible values that can be entered. If your company has three departments (research, marketing, and IS) you can build a plug-in for creating users that allows user creation only in those containers.