The MUX tick box should be checked only if you use a multiplexer in your design. The PMBUS host recognizes each device through its unique 7-bit slave address. As the number of I2C devices increases in the system, it's possible to have address conflicts. Sometimes several identical devices are needed in the same system. To avoid I2C address conflicts, an I2C multiplexer can be used to communicate to the PMBUS host correctly. If this is not the case leave the MUX tick box unchecked.