What is BuiltinTopicKey_t ?

[jhelovuo]

The DDS and RTPS specifications mention type BuiltinTopicKey_t. See e.g. Table 8.6 - RTPS Entity Attributes in RTPS Spec 2.5. Its exact contents, definition, and meaning are a bit mysterious.

Questions to be answered:

• What is the size of the type? Some documents say 3x 32 bits (12 bytes) and somewhere else it is 4x 32 bits (16 bytes).
• Is is the same as a GUID (16 bytes) or A GUID prefix (12 bytes)?
• This type appears in RTPS Spec "Figure 8.30 - Data types associated with built-in Endpoints used by the Simple Endpoint Discovery Protocol". It is used to identify a Topic. Who defines the BuiltinTopicKey_t for a Topic? Is it actually used somewhere?
• Is the BuiltinTopicKey_t for a Participant, Reader, and Writer the same as their GUID?
• What happens if an implementation ignores it?
• Is the BuiltinTopicKey_t somewhere communicated (indirectly) over RTPS?