CMIS-Core.xsd:

<xs:complexType name="cmisTypeRelationshipDefinitionType">
<xs:complexContent>
<xs:extension base="cmis:cmisTypeDefinitionType">
<xs:sequence>
<xs:element name="allowedSourceTypes" type="xs:string"
minOccurs="1" maxOccurs="unbounded" />
<xs:element name="allowedTargetTypes" type="xs:string"
minOccurs="1" maxOccurs="unbounded" />
</xs:sequence>
</xs:extension>
</xs:complexContent>
</xs:complexType>

However, part 1 (2.6.1.1) states:

allowedSourceTypes : If this attribute is "not set", then the source object MAY be of any type.
allowedTargetTypes : If this attribute is "not set", then the target object MAY be of any type.

Note: minOccurs was "0" in previous versions of the spec. I can't find a JIRA issue relating to the change.