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"!