Which protocol specifies the meaning of each bit in a CAN message?
This is specified by the application layer - refer to the &quot;OSI model&quot; where CAN itself only specifies hardware and data link layers. All nodes on the bus need to be aware of the application layer protocol used.
Common application layer standards are CANopen, J1939 and DeviceNet. In automotive systems, it is most common to use custom, OEM-specific protocols for control signals, though car OBD protocols specifically are somewhat well standardized (OBD2 etc).