Your warrant card is the card in your armband, and it has on it the terms and duties of a marshal, space to record your sessions, your details etd. I find the actual armband a nuisance as it slips off my arm, but I have been told that cutting the armband and sewing the holder to your tabard is not allowed... why don't we get credit card sized credentials like the ACU ones? Much handier, and maybe in the long term chipped.
I advocated this for licences, so they could be chipped and then record your races for, say, 5 years, removing the need for the silly 6 qualifying races rule... all places, and also misdemeanors would be recorded at every race (internationally?)
In the case of marshals you could build up a credit... like your Tesco card (not mine, I hate 'em, but that's another story!).
Come on... it's the 21st century now...