Icon indicia are provided on input, output and plug-in power processing units to permit proper matching of plug-in units with each other and with the input and output power processing units in a power distribution system. The indicia are in a co-ordinated matrix format with input icons on a designated chroma background to identify input voltages and output icons on a second designated chroma background to identify output voltages. Icons and background chroma on the plug-in power processing units are coordinated with related icons and background chroma at the same matrix position on other plug-in units and/or the input and output units to assure the compatibility of all the units. A craftsman inserting a plug-in unit onto a shelf of the power chassis need merely compare the unit's icon and chroma background indicia with the input, output, or other unit's icons and associated chroma background indicia to be assured of inserting the correct plug-in unit into the particular shelf.