The Regulation sets out an extensive catalogue of definitions used throughout the instrument, cross‑referencing Directive 2014/65/EU and other Union legislation. It defines investment firms, investment services and ancillary services, client categories, market structures and trading venues (regulated markets, MTFs, OTFs, systematic internalisers, market makers), classes of financial instruments (shares, bonds, transferable securities, derivatives, commodity and agricultural derivatives, ETFs, certificates, structured finance products), CCPs and clearing-related concepts, and data reporting and publication entities (APAs, ARMs, CTPs, data reporting services providers). It also defines market data concepts including ‘liquid market’, ‘core market data’ and ‘regulatory data’.

The Commission is empowered under Article 50 to adopt delegated acts to specify technical elements of those definitions and to adjust them to market developments. Separate delegated‑act powers permit the Commission to specify criteria to identify ARMs and APAs that, by derogation for limited internal‑market relevance, may be authorised and supervised by a Member State competent authority, taking into account factors such as single‑Member‑State service provision, numbers of trade reports and cross‑border group activity. Where an entity is supervised by ESMA as a data reporting services provider, its ARM or APA activities cannot be excluded from ESMA supervision by such delegated acts.