CAN Reference Output Voltage
The CAN bus needs to be terminated with \$120\Omega\$ in order to work properly. There are now two basic ways how this can be done. The easiest thing would be to connect \$120\Omega\$ between CAN_H and CAN_L. This has the drawback however, that the DC bias level of the bus is not defined properly. In order to have this achieved you have to use split termination, and this is where VREF becomes relevant.

According to the datasheet, VREF is somewhere around VCC/2. If you connect \$60\Omega\$ resistors from CAN_H and CAN_L to VREF you have established proper termination of the bus and furthermore you have a defined bias level of VREF/2 if the transceiver is in its recessive state.