A substitution group is used to use elements as placed holders.

It is a way to have an extensible "choice" in your content model.

Elements elect to "join" the substitution group by having a 'substitutionGroup' attribute:

<xs:element name=a" type="xhtml:Anchor" substitutionGroup="xhtml:inline"/>

The target, in this case 'xhtml:inline', is called the representative member of the substitution group.

The type of the element must be derived from the type of the representative member.