Description of Element Modifiers in DTDs

Table 59 describes the available modifiers:

Table 59. Element Modifiers
Modifier	Description	Indicator in DTD
Optional	This element can appear once or not at all. (0 or 1)	Question mark ( `?`)
Zero or more	This element is optional and repeatable. (0, 1, or more)	Asterisk ( `*`)
One or more	This element is required and repeatable. (1 or more)	Plus sign ( `+`)
Choice	Exactly one of the specified subelements must appear.	Vertical bar ( `\|`)
Sequence	If no other modifiers are defined on the `Sequence` modifier, each subelement in this element must appear exactly once. In other words, it is required. Also, the subelements must appear in the order in which they are specified in the referencing element. You can define other modifiers on the `Sequence` modifier. In this way, you can specify that some subelements are optional, some appear zero or more times, and some appear one or more times.	Comma ( `,`)