The field element specifies an XPath expression that specifies the value used to define an identity constraint.
(The ? sign declares that the element can occur zero or one time inside the field element)
|id||Optional. Specifies a unique ID for the element|
|xpath||Required. Identifies a single element or attribute whose content or value is used for the constraint|
|any attributes||Optional. Specifies any other attributes with non-schema namespace|
The following example shows a field element that specifies the "userID" attribute as the field to use for the identity constraint:
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