XML in a Nutshell, 3rd Edition

Sequences

In practice, a content specification that lists exactly one child element is rare. Most elements contain either parsed character data or (at least potentially) multiple child elements. The simplest way to indicate multiple child elements is to separate them with commas. This is called a sequence. It indicates that the named elements must appear in the specified order. For example, this element declaration says that a name element must contain exactly one first_name child element followed by exactly one last_name child element:

<!ELEMENT name (first_name, last_name)>

Given this declaration, this name element is valid:

<name>
  <first_name>Madonna</first_name>
  <last_name>Cicconne</last_name>
</name>

However, this one is not valid because it flips the order of two elements:

<name>
  <last_name>Cicconne</last_name>
  <first_name>Madonna</first_name>
</name>

This element is invalid because it omits the last_name element:

<name>
  <first_name>Madonna</first_name>
</name>

This one is invalid because it adds a middle_name element:

<name>
  <first_name>Madonna</first_name>
  <middle_name>Louise</middle_name>
  <last_name>Cicconne</last_name>
</name>