After Sheffield Bank has written the €15M cheques, it updated its balance to €45M in its books, but its account at BNPI bank would still show €60M while the cheque is clearing. The difference is a disbursement float of +€15M.
After Sheffield Bank has deposited the €24M cheque, in its book its bank account will show an updated balance of €69M, (i.e., €45M + €24M = €69M). However, its available funds at BNPI Bank plc don’t rise until the cheque clears, resulting in a collection float of −€24M.
Sheffield Bank’s net float is the sum of the collection and disbursement floats, i.e., +€15M - €24M or −€9M.