A scenario where a family or workplace or other organization has everyone fill in their ballot in front of each other and then drop it off in the mail together.
“For convenience,” “to make sure everyone gets their vote in.”
The difference is there is no designated, guaranteed-private voting booth for the act of voting.
I live in CA where we have had mail-in voting for some time. As far as I know, I don't have anyway to prove to someone that I voted a particular way.