This selects whether the BUC's LO frequency is added to or subtracted from the modem's Tx frequency to derive the final uplink RF frequency. Because Terminal Frequency = BUC_LO plus or minus Tx_Frequency depending on this setting, choosing the wrong mix produces a transmit frequency off by twice the Tx IF offset, potentially transmitting into an entirely different, occupied satellite channel. Selecting highSideMix(1) with a 13050 MHz LO and 1000 MHz Tx frequency yields 14050 MHz uplink, whereas lowSideMix(0) with the same numbers would instead target 12050 MHz.