I have a fairly new QO main svc panel (QO142M200PCAFVP) which is being used as a service panel (bonded, not a sub-panel). The green screw (top right) bonds the panel appropriately. The main 3 connectors are identified (120v/120v/neutral). There is a smaller lug next to the neutral lug which I don't see being utilized anywhere, marked in green "???" in my picture. It can take a 4GA or maybe slightly bigger wire.
What is the official role of that lug, and can I throw my outside ground wire (attached to an 8ft ground rod) on it?
Thank you very much for the explanation below, "ThreePhaseEel"!
The resulting service panel wiring is in the second picture.