The section Multiplicity in element documentation pages reports the occurence indicators of the respective element, by name the values of minOccurs and maxOccurs. For example, in <ocpRef> (railML® 2) the multiplicity is depicted with [2..2], which means, that minOccurs=maxOccurs=2 resp., every <IS:path> requires exactly two instances of <IS:ocpRef>.
The default values for minOccurs and maxOccurs are 1, which means that on default an element has to be used exactly once, like in <IS:trackTopology> (railML® 2) and in <IS:topology> (railML® 3).
As an element cannot be employed a negative number of times or halve a time, minOccurs and maxOccurs have to be non-negative integers. maxOccurs must be ≥minOccurs. maxOccurs may be unbounded (∞/*), as with <track>.
For Further Reading
- https://www.w3schools.com/xml/schema_complex_indicators.asp (external link)