1.2. Notation of a quantifier

The following table describes the explicit forms of a quantifier:

occurs n     exactly n occurrences
occurs m..n  minimum m occurrences, maximum n occurrences
occurs n..*  minimum n occurrences, maximum unlimited

Alternative notations:

occurs required exactly one occurrence (occurs 1)
occurs optional zero or one occurrence (occurs 0..1)
occurs ?        zero or one occurrence (occurs 0..1)
occurs *        unlimited occurrence (occurs 0..*)
occurs +        one or more occurrences (occurs 1..*)

Note since the default value of the quantifier is "required" it may be omitted. E.g.:
   ISBN="required int(10000000, 999999999)" can be written as ISBN="int(10000000, 999999999)"