The Unique Particle Attribution (UPA) rule is XML schema's mechanism to prevent schema ambiguity.

Due to the UPA rule the schema fragment given below is prohibited.

   <xsd:element name="x" type="xsd:integer" minOccurs="0"/>
   <xsd:any minOccurs="0" maxOccurs="unbounded" processContents="skip"/>

Given the instance fragment:


It is not possible to create a Post-Schema-Validation Infoset, because it is ambiguous whether <x> should be associated with the element declaration x, or the wildcard (xsd:any).

The W3C schema workgroup is considering weak wildcards for schema version 1.1. Using weak wildcards, the explicit element declaration would always take precedence (<x> is associated with the element declaration), thus removing the ambiguity.

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