USIs were introduced in late 2012 in the U.S. in the context of Dodd–Frank regulation, where reporting of transactions to Trade Repositories first became mandatory.
European financial market regulations followed suit, with reporting to Trade Repositories under EMIR requiring UTIs from February 2014 on.
LEI codes are used across the financial reporting regimes in the U.S. (Dodd-Frank) and Europe (EMIR / MiFID).
The current recommendation by the BIS, based on the CPMI/IOSCO Harmonization Group consultations regarding the prefix (the mint component of the UTI), is: "With regard to what code should constitute the mint component, the CPMI and IOSCO have applied the preference for using existing international standards and have selected the LEI code".
The CFTC defines the transaction identifier part as an alphanumeric code of variable length, up to a maximum of 32 characters.