Regulatory Technical Standards: the detailed binding rules that specify how a piece of EU legislation must be applied in practice.
In open banking, the RTS on strong customer authentication and secure communication set out how SCA works, which exemptions exist, and what a bank's interface for third parties must do. They are drafted by the European Banking Authority and adopted by the Commission.
The distinction matters because the directive states the principle while the RTS determine what a provider actually has to build.
