Hi,
please refer to chapter 55.3.3 Arbitration process of the RM for the explanation of transmitting priority.
During arbitration process the TX MB winner is selected using below mechanisms depending on CTRL1[LBUF] and MCR[LPRIOEN] setting.
- Lowest-number mailbox first or
- Highest-priority mailbox first
For example the SDK has no support/option to configure that, CTRL1[LBUF] is keeping cleared, I think.
BR, Petr