Something kind of like this
<?xml version="1.0" encoding="ISO-8859-1" ?>
<xs:schema xmlns:xs="http://www.w3.org/2001/XMLSchema">
<xs:complexType name="Client">
<xs:sequence>
<xs:element name="ClientID" type="xs:integer"/>
</xs:sequence>
</xs:complexType>
<xs:complexType name="Process">
<xs:sequence>
<xs:element name="Name" type="xs:string"/>
<xs:element name="Active" type="xs:boolean"/>
<xs:element name="Process_Client" minOccurs="0" maxOccurs="1">
<xs:complexType>
<xs:sequence>
<xs:element name="Client" type="Client" maxOccurs="unbounded" minOccurs="1"/>
</xs:sequence>
</xs:complexType>
</xs:element>
</xs:sequence>
</xs:complexType>
<xs:element name="Process" type="Process"/></xs:schema>
The key is when you set maxOccurs and minOccurs. This defines the multiplicity of the relationship. Here I am defining the association from Process to Client as many-to-one (many Clients for one Process).