---
title: "Unique Particle Attribution Constraint | Microsoft Docs"
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---
# Unique Particle Attribution Constraint
[!INCLUDE[appliesto-ss-asdb-xxxx-xxx-md](../../includes/appliesto-ss-asdb-xxxx-xxx-md.md)]
  In XSD, complex content models are constrained by the unique particle attribution (UPA) constraint rule. This rule requires that each element in an instance document correspond unambiguously to exactly one `<xsd:element>` or `<xsd:any>` particle in its parent's content model. Any schema that contains a type with a potentially ambiguous content model is rejected.  
  
 The most common causes of ambiguity are `<xsd:any>` wildcard characters and particles that have variable occurrence ranges, such as minOccurs < maxOccurs. For example, the following content model is ambiguous, because an <`e1`> element could match either the `<xsd:element>` or the `<xsd:any>` element.  
  
```  
<xsd:element name="root">  
    <xsd:complexType>  
        <xsd:choice>  
            <xsd:element name="e1"/>  
            <xsd:any namespace="##any"/>  
        </xsd:choice>  
    </xsd:complexType>  
</xsd:element>  
```  
  
 The following content model is also ambiguous:  
  
```  
<xsd:element name="root">  
    <xsd:complexType>  
        <xsd:sequence>  
            <xsd:element name="e1" maxOccurs="2"/>  
            <xsd:element name="e2" minOccurs="0"/>  
            <xsd:element name="e1"/>  
        </xsd:sequence>  
    </xsd:complexType>  
</xsd:element>  
```  
  
 Although a document such as `<root><e1/><e2/><e1/></root>` can be validated unambiguously, a document such as `<root><e1/><e1/></root>` cannot, because it is not clear to which `<xsd:element>` the second `<e1/>` corresponds. Even though some documents can be validated unambiguously, the schema will be rejected, because of the potential for ambiguity.  
  
 Note that for a content model to be valid, it must be possible to validate any instance unambiguously without looking ahead. For example, consider the following content model:  
  
```  
<xsd:element name="root">  
    <xsd:complexType>  
        <xsd:choice>  
           <xsd:sequence>  
               <xsd:element name="e1"/>  
               <xsd:element name="e2"/>  
           </xsd:sequence>  
           <xsd:sequence>  
               <xsd:element name="e1"/>  
               <xsd:element name="e3"/>  
           </xsd:sequence>  
       </xsd:choice>  
    </xsd:complexType>  
</xsd:element>  
```  
  
 For a document such as `<root><e1/><e3/></root>`, the sequence `<e1/><e3/>` unambiguously matches the second `<xsd:sequence>`. However, because the `<xsd:element>` to which `<e1/>` corresponds cannot be determined without looking ahead to `<e3/>`, the content model violates the UPA constraint rule.  
  
## Finding More Information  
 The following document is published by the World Wide Web Consortium (W3C) and contains the technical description of the unique particle attribution constraint:  
  
 "XML Schema Part 1: Structures Second Edition, W3C Proposed Edited Recommendation":  
  
-   Section 3.8.6: Constraints on Model Group Schema Components  
  
-   Appendix H: Analysis of the Unique Particle Attribution Constraint (non-normative)  
  
 To see the document, visit [http://www.w3.org/TR/xmlschema-1](https://go.microsoft.com/fwlink/?linkid=48881).  
  
## See Also  
 [XML Schema Collections &#40;SQL Server&#41;](../../relational-databases/xml/xml-schema-collections-sql-server.md)