It doesn't, I have seen cases where a customer has attempted to travel on a ticket that had been used on the PDF, but approached the barrier & having told that it's been used stated that he had only just activated it so how could it be used?They are and they aren't.
For Avanti and LNR/WMT, you are sent an e-mail with a PDF which is a proper e-ticket. You can then also download it via the app which creates something that looks and acts very much like an m-ticket (requiring activation and barring refund if you do activate it). However, you can still, so far as I know, use the e-mailed PDF if you prefer, I don't *think* (but haven't tried) that downloading it into the app cancels the e-ticket, indeed I think it's the same barcode.
Sam