---
type: "sdc-page"
title: "SDC: StructureDefinition-sdc-codesystem-intro"
source: "sdc/input/pagecontent/StructureDefinition-sdc-codesystem-intro.xml"
---
# SDC: StructureDefinition-sdc-codesystem-intro

### Scope and Usage

Code systems are referenced by ValueSets to define possible answer choices in a re-useable cross-questionnaire (and other data structure) manner. Codes might be drawn from standard international terminologies like LOINC or SNOMED CT or from regional code systems for drugs, billing codes, etc. The use of standardized codes is useful when data needs to be populated into the questionnaire or extracted from the questionnaire for other uses. It is also helpful when the set of possibilities is highly dynamic - for example drug codes where the set of available codes might change daily or weekly.

This profile sets expectations for use of the [CodeSystem]({{site.data.fhir.path}}codesystem) resource when used as a source for answers to questions in a Questionnaire. It identifies which core elements and extensions must be supported and highlights other elements that, while not required, may be relevant to the Questionnaire use-case.

For systems where discovery of CodeSystem is important, implementers are encouraged to also comply with the [CRMI Shareable CodeSystem](http://hl7.org/fhir/uv/crmi/StructureDefinition/crmi-shareablecodesystem) profile.