Which Field is missing ('?') from this CAN Datagram?

Bit-field layout of a CAN data frame. The upper horizontal frame is shown from Bus Idle and SOF through a blue Arbitration Field, yellow Control Field, red Data Field, CRC Sequence, CRC delimiter (DEL), ACK and ACK delimiter, EOF, ITM, then Bus Idle. Below are enlarged "Standard Format" and "Extended Format" layouts. The standard row shows a blue Identifier, RTR, IDE, r and a yellow rectangular field with a red question mark at the end of the control field. The extended row shows Base Identifier, SRR, IDE, Extended Identifier, RTR, r1, r0 and another yellow rectangle marked with a red question mark. Dashed vertical guides map the subfields to the upper frame. No bit count is supplied for the question-marked fields.