Others may know better, but my guess is shift allowance will be at that amount if you work your normal roster and don't swap shifts with others, fo example night shifts into day shifts, and if you do, you lose allowance for that swapped shift.
Family Rewards Benefit Scheme is their name of employee benefit scheme, which most rail companies offer, but every company has their own name for it, there's diffrent providers of that website, and diffrent features that may be provided to you. It's an online website to which you get access once you're employed by them.
Features available for DB: Employee discounts (cashback for online shopping, discounted gift cards to many shops including Tesco, Sainsburys, M&S), Cycle to Work scheme and Childcare Vouchers.
