Hi @pb632146,
Is that the same project I referenced previously? It has some differences in that function, but that part simply sets up the TJA1153 transceiver. You can find a brief description in the setupCanXCVR function:
"The TJA1153 transceiver does not come ready to use as most CAN transceivers
It enters configuration mode initially if it has not been configured before,
in this initial state, it is ready for receiving additional security
setups such as an ID blocklist for example.
In this example, the ID being transmitted is added to the CAN0 transceiver's
passlist, other configurations are left as default.
Refer to the NXP's TJA1153 data sheet for further detail."
Here is the configuration for the project:


For the IRQs, you can see them declared in the FlexCAN_Ip_IRQ.c file:


And finally, for the FlexCAN MB, each individual message buffer is 16, 24, 40, or 72 bytes, depending on the quantity of data bytes allocated for the message payload: 8, 16, 32, or 64 data bytes, respectively. Please refer to chapter 73.6.3 from the S32K3 Reference Manual.
Best regards,
Julián