I think those are the additional “quick slot” a multicrew gunner has access to in the game. Per the manual: “Gunner: Gunner takes control of all turreted weapons, missile launchers, counter measures, advanced scanners and limpets. Gunner can use the fire group interface to set up fire groups for these modules. Here, the gunner has access to two additional quick slot buttons, which can be assigned to any single module under gunner’s control.”
regards