MRG messaging clarification on message id

Issue

• MRG messaging clarification on message id
• The message id is generated by the broker when a message is sent to the broker. Right ?
• What are the uniqueness properties of the message id ?
• Can any two messages sent to the same broker have the same message id ?
• What about messages in a federation ? Can any two messages sent to different brokers in the federation have the same message id ?
• What is the format of the message id ? Looks like it is a UID