What data type contains components for both a locally defined code and an external coding system?

The data type that contains components for both a locally defined code and an external coding system is the HD (Hierarchical Data) type. This data type is specifically designed to represent information that can include identifiers for both local and external reference systems.

In the context of HL7, HD is utilized when there is a need to handle a coding system that may be defined by the local context (such as a facility-specific code) while also having the capability to link to established external coding systems (like SNOMED, LOINC, or ICD). The structure of HD allows for encapsulating the necessary details to ensure compatibility and interoperability between different health information systems, promoting better data exchange and integration.

The other data types listed, while relevant within HL7, serve different functions. CE (Coded Element) is primarily used for a coded value that might not specify the coding system context in the same way as HD. ID (Identifier) is for a simple identifier without the need for external codes, and IS (Coded String) is used to represent coded information, but it does not contain the components needed to specifically refer to both local and external coding systems in the structured manner that HD does.