The subgroups on our board are a little confusing but hopefully this will clear up any confusion.
The way our current board is setup is to use the subgroups as "volumes" for different groups on the board, for instance, there is a group for Band, Praise, Choir, Wireless. If you have the choir "mixed" to what you think sounds pretty good but still aren't getting enough volume, you can raise the Red subgroup for "Choir" and it will boost the overall choir sound.
This is good practice in "mixing" sound to try and pull the band up on instrumentals or solos and the choir or wireless up on different solo parts.
Below is a picture of the subgroups for reference. Each section has two slides, they need to be moved in tandem (together).