A FHIRPath expression or FHIR Query that resolves to a list of candidate answers for the question item or that establishes context for a group item. The user may select from among the candidates to choose answers for the question.

The results of the expression must correspond to the item type of the question the element appears on or must correspond to a resource, backbone element or complex data type for a group item. Resources are considered a match for the Reference item type. Quantity can be a match for decimal and integer items so long as a questionnaire-unit extension is present.

Unique id for the element within a resource (for internal references). This may be any string value that does not contain spaces.

May be used to represent additional information that is not part of the basic definition of the element. To make the use of extensions safe and manageable, there is a strict set of governance applied to the definition and use of extensions. Though any implementer can define an extension, there is a set of requirements that SHALL be met as part of the definition of the extension.

There can be no stigma associated with the use of extensions by any application, project, or standard - regardless of the institution or jurisdiction that uses or defines the extensions. The use of extensions is what allows the FHIR specification to retain a core level of simplicity for everyone.

Source of the definition for the extension code - a logical name or a URL.

The definition may point directly to a computable or human-readable definition of the extensibility codes, or it may be a logical URI as declared in some other specification. The definition SHALL be a URI for the Structure Definition defining the extension.

Value of extension - may be a resource or one of a constrained set of the data types (see Extensibility in the spec for list).