I'm getting confused by the FlexCAN documentation. This is for a KE18F part. The documentation says that there are 16 message buffers, but 64 mailboxes. If I'm using the Rx FIFO, then message buffers 0-7 are not available. The confusing part is that it seems to use the terms "Message Buffer", "Mailbox", and "MB" interchangeably. What do mailboxes 17-64 represent then, and what are they used for?