None. The main principle which applies to reimbursement is that bus operators are ‘no better/no worse off’ as a result of taking part in concessionary fares schemes. Reimbursement for lost revenue should be based on the average amount each concessionary traveller would have paid in the absence of the scheme (the average equivalent full fare).
It is for each Concessionary Travel Authority and its bus operators to reach local agreement on the appropriate “average fare”. Reimbursement payments are subject to audits of ticket sales data and, if the authority requires further information, on-bus surveys of patronage.