Substitution groups are a flexible way to designate element declarations as substitutes for other element declarations in content models. You can easily designate new element declarations as substitutes, from other schema documents and even other namespaces, without changing the original content model. This chapter describes how to define and use substitution groups.
Substitution groups are useful for simplifying content models, making
choice groups more flexible, and allowing more descriptive element-type names to be used, including localized names.
Suppose you have a section of a purchase order that lists products of various kinds. You could use repeating ...