Trading venue transaction identifier
Assigned by the trading venue to a transaction. In the context of ESMA RTS 22 and RTS 24, this is an unique transaction identification
number generated by trading venues and disseminated to both the buying and selling parties in accordance with Article 12 of [RTS 24 on the maintenance of relevant data relating to orders in financial instruments under Article 25 of Regulation 600/2014 EU]. (quoted text from RTS 22).
Uniqueness may be defined per relevant regulations.