Mix VS Groups Noob Question

I am setting up a QU-7 for the first time and am wondering what the difference is between Mixes and Groups.

I have read the manual and it does not seem to explain the difference.

I have searched the forums and haven’t seen a post that answers my question.

When setting up the vocal channels I attempted to place them into a group, and assign that group to the Mix7 button. but when I recall another Mix button, then return to my vocal group, assigned as a group to the Mix7 button, the assignment of the vocal channels is gone. The faders are not where I had set them.

In the meantime I have just set up the vocals to be assigned to a mix, and that works as expected: I recall a different Mix button, go back to the vocal mix and the faders return to where I had originally assigned them.

Thanks for your help hive mind.

66Pbass