QO Main Service Panel - Ground lug question - CT resident
That lug is intended to be used as a grounding electrode conductor termination, but can be used as an extra neutral instead
The Square-D neutral design basically intends for the grounding electrode conductor (&quot;outside-ground-rod wire&quot; or water pipe bond wire) to be landed on the N-G strap at the lug you point out, with the larger lug next to it used for the service neutral, and the bond to the panel cabinet provided by the green screw through the neutral intertie crossbar; this keeps all the busbar holes provided by the panel free for branch neutrals and grounds.
However, since you have removed the bonding screw, and are using a wire as your bonding jumper instead, NEC 250.24(D) permits you to do as Harper describes and use the left-hand equipment grounding bar to land your grounding electrode conductor instead.  As a result of this, you can now use the lug you point out as a spare branch neutral lug to make up for losing a branch neutral hole to the bonding jumper.  Doing as you and the manufacturer intend and using the small lug on the neutral strap for the grounding electrode conductor is fine still, though.