DDS15 — Spec lacks definition regarding uniqueness of InstanceHandle_t

The spec defines the following:
7.1.2.1.1.8 get_instance_handle
This operation returns the InstanceHandle_t that represents the Entity.

But it doesn't specify in how far this InstanceHandle_t has to be unique. When the user receives an InstanceHandle_t, is it than unique within the same domain participant, within the process or within the complete dds domain? If it is for example just unique within one domain participant, we can't use it for example as key in a map which could contain entities from multiple domain participants