A representative number comes in the pattern 'XDD' where 'X' can be a letter or number and 'D' must always be a number. Letters are case senstitive and must be in uppercase to be valid.

In the form below 'Rep. number 1' uses the extension function to format any letters in the value in uppercase to make it valid, 'Rep. number 2' does not and therefore is only valid if you enter any letters in uppercase.