Cardholder Data Environment (CDE)
The scope boundary under PCI-DSS: systems that store, process or transmit cardholder data.
The CDE is the people, processes and systems that store, process or transmit cardholder data, plus connected systems. Scoping the CDE narrowly (e.g. via a token vault) is the single biggest lever for reducing PCI-DSS cost. Everything inside the CDE is in scope for the standard's 12 requirements and 300+ sub-controls.