A Mix is an auxiliary mix of channels to be sent somewhere separate from the Mains mix. For example, as monitor mixes or for an FX bus. A Group is a bus for manipulating multiple channels together. For example, you could group all drum channels together then EQ/compress/control them all together for a more coherent sound, and then send that group into the LR mains rather than the individual channels. On the Qu, Groups can be switched to Mix mode. A Matrix is a mix of mixes. For example, you could take your mains output and add some boost from that drum group to create a matrix for recording.