The MUXBUS is the bus between the FPGA on the macrocontroller to communicate with the offboard CPLD. With the TS-8100, the CPLD includes all of the DIO and PC104. The MUXBUS config register in the #Syscon allows enabling and configuring the speed for this bus. The MUXBUS timing also influences the communication with PC104 peripherals.
For more advanced details on the MUXBUS, refer to the implementation details here.