A substitution group is similar to a complex content type in that it allows derivations from a common base. In this case, however, the base is an XSD element and the substitution group allows any of a set of elements defined to be in the group to be used in the place of the base element. A simple example of this is as follows:

   <xsd:element name="MyElement" type="MyType"/>
   <xsd:complexType name="MyType">
      <xsd:sequence>
         <xsd:element ref="MyBaseElement"/>
      </xsd:sequence>
   </xsd:complexType>
   <xsd:element name="MyBaseElement" type="xsd:string"/>
   <xsd:element name="MyExtendedElement" type="xsd:string" substitutionGroup="MyBaseElement "/>

In this case, the global element MyElement references MyType which is defined as a sequence with a single element reference to MyBaseElement. MyBaseElement is the head element in a substitution group that also includes MyExtendedElement. This means MyType can either reference MyBaseElement or MyExtendedElement.

As per X.694, ASN1C generates a special type that acts as a container for all the different possible elements in the substitution group. This is a choice type with the name <BaseElement>_group where <BaseElement> would be replaced with the name of the subsitution group head element (MyBaseElement in this case).

The generated C type definitions for the above XSD definitions follow:

   typedef const OSUTF8CHAR* MyBaseElement;

   typedef const OSUTF8CHAR* MyExtendedElement;

   #define T_MyBaseElement_group_myBaseElement 1
   #define T_MyBaseElement_group_myExtendedElement 2

   typedef struct EXTERN MyBaseElement_group {
      int t;
      union {
         /* t = 1 */
         MyBaseElement myBaseElement;
         /* t = 2 */
         MyExtendedElement myExtendedElement;
      } u;
   } MyBaseElement_group;

   typedef struct EXTERN MyType {
      MyBaseElement_group myBaseElement;
   } MyType;

   typedef MyType MyElement;

In this case, if MyElement or MyType is used, it can be populated with either base element or extended element data.