I think it is sensible to assume that whilst they are to be branded and used exclusively for Service 7, no depot will leave a journey missing if they have a spare bus - regardless of what brand it is.
The difference is that some depots are better at actually getting the buses on the right routes from first thing and the confidence that they will stay on there and not require to come off for work.
RE: Arriva North East: Latest News & Discussion - April 2015