(18)To avoid conflicts between the originator and the investor, and to ensure legal certainty in terms of the scope of the credit protection purchased for underlying exposures, such credit protection should reference clearly identified reference obligations, giving rise to the underlying exposures, of clearly identified entities or obligors. Therefore, the reference obligations on which protection is purchased should be clearly identified at all times, via a reference register, and kept up to date. That requirement should also be indirectly part of the criteria defining the STS on-balance-sheet securitisation and excluding arbitrage securitisation from the STS framework.