Reserve bit to distinguish

The reserve bit of the traditional CAN frame now becomes the switch for the CAN FD format. If transferred dominant as 0, it denotes a classical CAN frame. If it contains the recessive value 1, it denotes a CAN FD frame.

FDF - Flexible Data Rate Format

The bit's new name is FDF for Flexible Data Rate Format which establishes the opportunity to transmit a much larger payload. The actual length of the data field and the decision whether to switch to a higher transmission rate or not, follow later in the frame.