This class defines a interface which XCube_ActionForm calls the check functions. But this class is designing now, you should not write a code which dependents on the design of this class. We designed this class as static method class group with a reason which a program can not generate many instance quickly. However, if we will find better method to solve a problem, we will change it.
Don't use these classes directly, you should use XCube_ActionForm only. This is 'protected' accesser in the namespace of XCube_ActionForm.