This selects whether the LNB's LO frequency is added to or subtracted from the incoming satellite frequency to derive the modem's receive IF frequency. Because the wrong mix flips the sign in Terminal Frequency = LNB_LO plus or minus Rx_Frequency, mismatching this against how the LNB actually operates causes the modem to search for the receive carrier at completely the wrong frequency and never lock. If the LNB is a standard low-side-mix Ku-band unit but this is set to highSideMix(1), the modem would compute the wrong expected Rx frequency and likely fail to acquire lock on an otherwise healthy signal.