XML in a Nutshell, 3rd Edition

Choices

Sometimes one instance of an element may contain one kind of child, and another instance may contain a different child. This can be indicated with a choice. A choice is a list of element names separated by vertical bars. For example, this declaration says that a methodResponse element contains either a params child or a fault child:

<!ELEMENT methodResponse (params | fault)>

However, it cannot contain both at once. Each methodResponse element must contain one or the other.

Choices can be extended to an indefinite number of possible elements. For example, this declaration says that each digit element can contain exactly one of the child elements named zero, one, two, three, four, five, six, seven, eight, or nine:

<!ELEMENT digit
 (zero | one | two | three | four | five | six | seven | eight | nine)
>