From new research by Jørgen Elklit and Michael Maley published at the Journal of Democracy:

Human-rights declarations and various legal instruments state that elections must be conducted by secret ballot, but are silent about precisely what that means. This paper discusses two situations in long-established democracies—the 2018 Swedish general elections and the 2017 Australian “plebisurvey “ on same-sex marriage—in which recognized principles of electoral secrecy were clearly violated.

It then identifies the main objectives of ballot secrecy (protecting the privacy of personal political beliefs, discouraging coercion, and preventing vote-buying) and the main measures needed to ensure them. Finally, it concludes that “secrecy” cannot be measured on a unidimensional scale.