XML in a Nutshell, 3rd Edition

Using Groups

Just as the xs:attributeGroup element allows commonly used attributes to be grouped together and referenced as a unit, the xs:group element allows sequences, choices, and model groups of individual element declarations to be grouped together and given a unique name. These groups can then be included in another element-content model using an xs:group element with the ref attribute set to the same value as the name attribute of the source group. When you do this, any occurrence constraints have to be specified on the reference to the group rather than on the definition of the group.