(13)Effective market surveillance in the case of a transaction in a combination of financial instruments presents particular challenges for market surveillance. The competent authority needs to have the global view and needs to be able to see separately the transaction in respect of each financial instrument that is part of a transaction in which more than one financial instrument is involved. Therefore, investment firms which execute transactions in a combination of financial instruments should report the transaction for each financial instrument separately and link those reports by an identifier that is unique at the level of the firm to the group of transaction reports related to that execution.