(17)Systematic internalisers should be defined as investment firms which, on an organised, frequent, systematic and substantial basis, deal on own account when executing client orders outside a regulated market, an MTF or an OTF. In order to ensure the objective and effective application of that definition to investment firms, any bilateral trading carried out with clients should be relevant and criteria should be developed for the identification of investment firms required to register as systematic internalisers. While trading venues are facilities in which multiple third party buying and selling interests interact in the system, a systematic internaliser should not be allowed to bring together third party buying and selling interests in functionally the same way as a trading venue.