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

Additional details on the use of this extension can be found on the [Form Behavior](/sdc-behavior#candidateExpression) and [Questionnaire Population](/sdc-populate#expression-based-population) pages. For further guidance on `candidateExpression` refer to the [Expressions page - candidateExpression](/sdc-expressions#candidateExpression) section.