[EDIT] I think I read about it in the context of correcting a common misconception or mistake, actually. Correct adjustment of the mirrors was described as watching a car leave your rearview mirror, and ensuring they show up on your sideview just as they're leaving the rearview—so if you can see cars on your sideviews before they're starting to slip off the edge of your rearview, you've got those adjusted too far in. For me, given how far back I put my seat, this ended up meaning I should adjust the mirrors all the way out in most cars. The idea is both to not have a gap in the sideview/rearview coverage, while maximizing side coverage within that constraint. So, if you can see cars in your sideview while they're still solidly in your rearview, that means you've got less side coverage than would be ideal.
Referenced in: https://www.caranddriver.com/features/a15131074/how-to-adjus...
(edited to include 2nd link)