This part of ISO 15926 specifies the initial set of core reference data items which can be used to re- cord information about process plants, including oil and gas production facilities. The following are within the scope of this part of ISO 15926: — core classes for process plants, including oil and gas production facilities; NOTE 1 Reference data items can be core classes, de facto classes, commodity classes and manufactured product classes. Reference data items can also be standard classes or proprietary classes. The terms for the different types of class are defined in 3.2. A discussion about the different types of classes is contained in Annex D. — the unique name for each reference data item; — the definition of each reference data item; — subclass and classification relationships between reference data items; NOTE 2 Each reference data item that is a class is directly or indirectly a subclass of an entity in ISO 15926-2. — the entity within ISO 15926-2 which can be used to record each reference data item. The following are outside the scope of this part of ISO 15926: — data requirements for additional reference data; — a numeric identifier for each reference data item; NOTE 3 Numeric identifiers may be assigned by a registration authority. — the procedures to be followed for registration and maintenance of additional reference data. NOTE 4 A core class defined by this part of ISO 15926 can be used by ISO 15726-2, ISO/TS 15926-7 or ISO 10303-221.