XML Schema minOccurs / maxOccurs default values

The default values for minOccurs and maxOccurs are 1. Thus:

<xsd:element minOccurs="1" name="asdf"/>

cardinality is [1-1] Note: if you specify only minOccurs attribute, it can't be greater than 1, because the default value for maxOccurs is 1.

<xsd:element minOccurs="5" maxOccurs="2" name="asdf"/>

invalid

<xsd:element maxOccurs="2" name="asdf"/>

cardinality is [1-2] Note: if you specify only maxOccurs attribute, it can't be smaller than 1, because the default value for minOccurs is 1.

<xsd:element minOccurs="0" maxOccurs="0"/>

is a valid combination which makes the element prohibited.

For more info see http://www.w3.org/TR/xmlschema-0/#OccurrenceConstraints

_{New, expanded answer to an old, commonly asked question...}

Default Values

<xsd:element name="A"/>

means A is required and must appear exactly once.

<xsd:element name="A" minOccurs="0"/>

means A is optional and may appear at most once.

 <xsd:element name="A" maxOccurs="unbounded"/>

means A is required and may repeat an unlimited number of times.

 <xsd:element name="A" minOccurs="0" maxOccurs="unbounded"/>

means A is optional and may repeat an unlimited number of times.

W3C XML Schema Part 0: Primer

In general, an element is required to appear when the value of minOccurs is 1 or more. The maximum number of times an element may appear is determined by the value of a maxOccurs attribute in its declaration. This value may be a positive integer such as 41, or the term unbounded to indicate there is no maximum number of occurrences. The default value for both the minOccurs and the maxOccurs attributes is 1. Thus, when an element such as comment is declared without a maxOccurs attribute, the element may not occur more than once. Be sure that if you specify a value for only the minOccurs attribute, it is less than or equal to the default value of maxOccurs, i.e. it is 0 or 1. Similarly, if you specify a value for only the maxOccurs attribute, it must be greater than or equal to the default value of minOccurs, i.e. 1 or more. If both attributes are omitted, the element must appear exactly once.

W3C XML Schema Part 1: Structures Second Edition

<element
  maxOccurs = (nonNegativeInteger | unbounded)  : 1
  minOccurs = nonNegativeInteger : 1
  >

</element>