A reference is an expressible sequence of names that identifies one specific declared item.

References are shaped like declarations (traversal, group, and item members), but a reference must name exactly one item, so its names can't contain expansions. Therefore, each name is a single LString (its words joined by spaces) instead of an LName.

Left Pane                       // item = "Left Pane"

End.Position                    // group = "End"
                                // item = "Position"

End.                            // Invalid reference; should have an item

Window/(Left,Right) Pane        // Invalid reference; which pane?

Cancel Button/Fill              // traversal = {"Cancel Button"}
                                // item = "Fill"

Window/Titlebar/Exit Button     // traversal = {"Window","Titlebar"}
                                // item = "Exit Button"

Button/Layout.Direction         // traversal = {"Button"}
                                // group = "Layout"
                                // item = "Direction"

The parser doesn't validate whether the item referenced has been declared or not; the engine does during resolution.