I think the consensus here is that you are supposed to use this spot to include any missed/different info than what is on AMCAS. Like suppose you are getting a publication in the fall, or you recently received a DUI that wasn't reported on AMCAS. Things like that.
-Important activities you accidently omitted from the Primary.
-Corrections of significant mistakes on the Primary.
-New activities just begun or significant updates on old ones.
-Explanations for GPA dips or irregular college attendance.
-Just accepted-for-publication manuscripts.
-Why this school or what makes you a perfect fit for this school.