Originally proposed by CiA in 2015, and completely revised in 2019, Signal Improvement Capability (SIC) is a technology to improve the quality of signals sent and received between devices on a CAN FD bus. This is done by suppressing the ringing caused by imperfect network topology. In CAN FD, arbitration ends after the arbitration segment, and the bit rate is no longer limited by arbitration, so it can be increased according to the specification (ISO 11898-1). During the data phase of a CAN FD frame the bit rate is limited by the network topology, and SIC improves the signal to allow a higher bit rate during the data phase.
Ringing is worst when the transmit signal goes from a zero to a one, or after a dominant to recessive transition. SIC can be implemented in the transmitter to suppress the ringing when transmitting, or in the receiver to filter the ringing when receiving. Both implementations enable an increased data bit rate, but they differ slightly in the details. SIC is not usually needed in Classical CAN because the bit rate is limited by arbitration and does not change during the data phase. Since Classical CAN does not increase the bit date after arbitration, SIC is not needed and would not add any benefits.
More information can be found at https://can-cia.org/can-knowledge/can/cia601/ and by downloading the CiA document 601 series: CAN FD guidelines and recommendations.