The constraints specified in the Graph Tool are exported in a constraints element of the graph element. Each constraint has its own tag, named after the type of constraint: connectivity, occurrence, uniqueness, portSame or portDifferent. The sub-elements of the constraints correspond to the fields in the definition dialog for that type of constraint.

<constraints>
  <connectivity>
    <object href="#..."/>
    <role href="#..."/>
    <max>1</max>
  </connectivity>
</constraints>

As the types mentioned in constraints will generally have been defined earlier in the graph element, their tags here will normally refer to them with href. Note though that the property in the port constraints is represented as a simple string, the local name of the property.