Flat Mirror
The flat mirror has a unity matrix and does not affect the propagating beam. It still is an essential element for round-trip matrix calculation when placed at the end of the resonator.
The flat mirror changes its representation on the layout when it does not locate at the end of the resonator. Similar to Point Element, you can use it as a helper element, e.g., as a separator between two adjacent range elements having different semantics.
See also
Elements Matrices, ../catalog, ../elem_props