Just to try and make it easier to understand, though Leyther Matt:
'Assign' on a network member would normally be enough to use to purchase an away tickets on priority during the season (or a Wembley ticket).
The manage option is to purchase a specific seat that has already been reserved at home for a ST holder by the system, to give ST holders the chance to guarantee their seat. Previous cup games this season were basically on open sale with ST Priority, so assign would work fine.
I've got my mate and his nearly 80 year old Dad, both on holiday in different places, and not due back until Saturday. My mate got his email to allow me to manage and accepted the request, all sorted within 10 minutes. His Dad is not contactable and probably can't remember what he has had for his breakfast, never mind the email address it might have been set up under. So I queued in the rain on a bank holiday for 1 hr 20 minutes to sort his out and had to pay 2 quid parking too. I think he's buying the drinks.