Each view in Revit is associated with a work plane. In some views (such as plan, 3D, and drafting) and for views in the Family Editor, the work plane is automatically set. In other views, such as elevation and section views, you need to set the work plane. The work plane is necessary for sketching operations such as creating an extruded roof, and for enabling tools in particular views, such as Rotate and Mirror in a 3D view.
When you set a work plane in a view, it is saved with that view. You can change the work plane as necessary.
When sketching, you can snap to the work plane grid, but you cannot align or dimension to it.