Higher bitrates can come at the cost of signal clarity which is why the Special Interest Group have devised the CAN XL Signal Improvement Capability (SIC) Transceiver. With this solution in place, CAN XL is able to bring new features to CAN communication such as now supporting payloads large enough to allow tunneling of ethernet traffic, known as “ethernet tunneling.” CAN XL will also add the ability to mix multiple higher layer protocols on the same bus. And it will do all of this with the reliability CAN is known for.

Like CAN FD before it, CAN XL is backwards compatible with CAN FD, allowing access to both CAN FD and Classical CAN protocols. While a CAN FD node cannot receive a CAN XL message, the node will not detect an error and will join arbitration on the next frame making it possible to use both CAN FD and CAN XL on the same bus. This is accomplished by taking advantage of CAN FD’s Protocol Exception State. This occurs when the recessive XLF bit of a CAN XL message sets the CAN FD node into the Protocol Exception State, a safeguard for future expansions introduced in the CAN FD specification, ISO 11898-1:2015.

Articles
CiA: Controller Area Network Extra Long (CAN XL)
Bosch: CAN XL Overview
CAN XL for real-time control and efficient file transfer - Kvaser Web Site 
CAN XL for real-time control and efficient file transfer - Whitepaper pdf