Custom Display is a special editing mode for making on-screen changes to the components of an MEP object in plan view. For example, you might want to add unique line work, such as vanes, hangers, and accessories to objects and fittings. Or, while working on a mechanical layout with piping, you might have a rack layout to which you want to add pipe breaks in order to see multiple elevations of the rack in a plan view. Finally, while placing equipment, such as electrical panels or base-mounted pumps, you might want to use a hidden line to indicate equipment pads or the access space required by the manufacturer around the equipment.
You can use Custom Display to edit objects that reside in externally referenced drawings (xrefs). When you save the changes, they do not affect the xref drawing itself. The custom display is valid only as long as the xref is associated with the same host drawing.
When the Custom Display mode is active, you can continue to add to other MEP objects in the drawing, but not to the object being edited. Also, you can use commands to add and delete linework for the object being edited.