DDS15 — InstanceHandle_t underspecified

In 2.2.2.1.1.8 the Instance handle is mentioned by the spec lacks some definition of InstanceHandle_t. In how far as instance handles unique and should they be comparable? Are they unique system wide or process wide? Is it legal to have two DDS entities (for example two domain participants) to have the same instance handle? This is important for users who for example store DDS entities in a std::map.