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