A reference cell is basically an empty physical native cell with a refcell property (property number 7150, as described in D.1). This property contains the information that ties the reference cell to a source and provides a bounding box. A complete example of a reference cell is shown below,
(Symbol asic2); (xic 4.2.9 LinuxRHEL7_64 03/01/2016 04:36 GMT); (PHYSICAL); (RESOLUTION 1000); ( CREATED 3/1/2016 4:36:34, MODIFIED 3/1/2016 4:36:34 ); 5 7150 filename="/usr/local/cad/layouts/asic2.gds" cellname=asic2 bound=0.000,0.000,2328.100,2543.700; 9 asic2; DS 0 1 1; DF; E
This reference cell is a stand-in for a cell named ``asic2'' which is found in the path given. Note the simple form of the cell, particularly realizing that the comment lines (enclosed in parentheses) are optional. It is completely feasible to create reference cells with a text editor. The only reason that the CHD was used is that it provides the correct cell bounding box. The bounding box is used in the display, but does not affect the actual location or size of the cell hierarchy when expanded.