Settings could depend on your current speakers but LFE is more common. As mentioned above, LFE+Main sends bass to both the sub and mains.
Note that there should be two settings. The setting for the LFE channel controls what gets sent to the subwoofer for low frequency effects (movie effects), so that is typically set at the highest setting of 120Hz. That's the .1 in 5.1 or 7.1 etcetera and the LFE channel only carries sub-120Hz material.
The crossover setting on the AVR controls the bass for the other channels (the 5 in 5.1). So material above the crossover setting goes to your main speakers and material below the crossover setting goes to the subwoofer. Most common setting is 80Hz but that can be adjusted depending upon the room and speakers. As mentioned above, the crossover setting on the subwoofer is set to maximum so that the AVR can control the crossover point. The worst thing a person can do is set the crossover on the subwoofer to say 80Hz and then set the AVR crossover to something higher, like 100Hz. That effectively creates a notch filter which filters out everything between 80Hz and 100Hz.