Reference Planes can be set up in a Project environment to control placement of Components. The Level and Grid elements are essentially Project Reference Planes.
Most System and Loadable Family definitions assume that the element will be Reference Plane based. Some Loadable Families are Face based where a surface of an element becomes the working Plane.
Most Revit Family Templates offers a predefined set of Reference Planes within which to develop components.
The Templates also have Datums which are essentially horizontal reference planes to work on.
You can create additional Reference Planes to suit the geometry of the intended component. These planes can be inclined but always extend to infinity as a 'surface' so the annotated Plane defining 'line' has no starting point to pivot from. There is no way to stop the Plane sliding sideways in space when set at an angle.
For example the Metric Window.rft template has Ref. Level as the default working reference Datum plane.
In a Family Template you can add Reference Lines which are similar to Planes in that they define working Planes
but the end of the defining Reference Line can be controlled by fixing it in space and it can have pivot angles to other Planes (or Lines).
The length of the Line can also be controled by a driving Length dimension.
Reference Lines are also used in an Adaptive Component Family to control surfaces, edges and constructed geometry.
The Reference Line has four (4) working planes for each Line, two at right angles to the Line and one at each end. This provides greater control over your solid geometry created in the definition.