The JESD204C 64B66B core operates using a 64-bit (8-byte) datapath. The core clock frequency is always the line rate divided by 66. For example, for a serial line rate of 16.5 Gbps, the core clock frequency is 250 MHz.
The AXI4-Stream RX/TX Data and Cmd interfaces operate at this core clock frequency. TX and RX core clock should be used as the clock source for these interfaces.