(25)Some payment instruments at domestic level enable the payer to initiate card-based payment transactions that are not distinguishable as debit or credit card transactions by the payment card scheme. The choices made by the cardholder are unknown to the payment card scheme and to the acquirer; as a consequence, the payment card scheme does not have the possibility of applying the different caps imposed by this Regulation for debit and credit card transactions, which are distinguishable on the basis of the timing agreed for the debiting of the payment transactions. Taking into account the need to preserve the functionality of the existing business models while avoiding unjustified or excessive costs of legal compliance and, at the same time, considering the importance of ensuring an adequate level playing field between the different categories of payment cards, it is appropriate to apply the same rule provided by this Regulation for the debit card transactions to such domestic ‘universal cards’ payment transactions. Nevertheless, a longer time period for adaptation should be left to those payment instruments. Therefore, by way of exception and during a transition period of 18 months after the entry into force of this Regulation, Member States should be able to define a maximum share of domestic ‘universal cards’ payment transactions which are considered as being equivalent to credit card transactions. For example, the credit card cap could be applied to the defined share of the total value of the transactions for merchants or acquirers. The mathematical result of the provisions would then be equivalent to the application of a single interchange fee cap on domestic payment transactions carried out with universal cards.