Cut and paste this URL to share the unmodified register and value:
https://regviz.com/r/STMicroelectronics/STM32H7x5_CM7/FDCAN1/FDCAN_RXF1A#0x0
FDCAN Rx FIFO 1 Acknowledge Register
Rx FIFO 1 Acknowledge Index
https://github.com/cmsis-svd/cmsis-svd-data