Un substance numbering is an imdg requirement on road and rail tanks freight vehicles freight wagons and containers and is displayed by either the use of diamond shaped placards 250 x 250 minimum size with black 65mm high numerals or an orange rectangular panel 300 x 120mm with 65mm high numerals as in centre margin nb when the latter method is used the panel must be accompanied by the appropriate hazard diamond with class number as shown in the imdg range