REST API Board IDs

Hi,

I am slightly confused by the board IDs being returned by the API. If I return a threaded list of messages, each message node has a <board> child node with a type attribute of 'board'  but also a <board_id> child node. What is this other board_id ? My call looks like:

/restapi/vc/boards/id/boardname/messages/threaded?&restapi.response_style=view

thanks!