The left of the three small buttons controls whether the band is in Dual Mono mode (off), Stereo Mode (Link on), or Mid/Side mode (on).
This toggle button can be set to two different states. On the left, it shows 'Left' (in blue) or 'Mid' (in orange). On the right, it indicates 'Right' (in blue) or 'Side' (in orange). When inactive the EQ is in dual-mono mode. You have independent control for the left and right channels.
When you click this button, it switches the chosen band. At the same time, it also changes the related band on the other channel.
For instance, if you click the mode button the matching band on the other channel will also switch.