@ Asker 16:30 Nov 5, 2014
As meir says, the text is on the face of it contradictory — they start by saying that a username always starst with a letter, and then that a 'person identifier' has 'leading digits' (i.e. NOT a letter!)
Either this extract of text doesn't give the whole story, or maybe it was written, by a non-native speaker of EN, who has confused 'digit' (= a figure) with 'character' (= ANY letter or figure sign)
In any case, it doesn't really make sense anyway: any string must inevitably have 'leading characters' — it has to start somewhere! We often talking about 'leading zeroes' (which may sometimes need to be chopped off, for example); unless it is referring to the absence of leading spaces — which is another issue entirely.
Of course, it is possible, as Meir hints, that this 'username' and 'person identifier' are not in fact the same thing at all. | 
peer agreement (net): +2