[elf_ears]

Full Public: Anything that involves being purposely exposed to people.

e.g posting photographs online, wearing humiliating clothes at a supermarket, making sexual sounds inside a toilet cubicle

Semi-Public? Anything that involved being in a public space but with little to no risk of being exposed.

e.g dares in changing rooms, secluded woods etc

Hidden Public? Anything that involves being in a public space but with no intention of being exposed.

e.g wearing a butt plug in public, masturbating beneath a table at a restaurant, also encompasses most semi-public dares

I wrote my answer before looking at the other responses so they would not influence what I wrote. Looks like we pretty much agree although I would clarify that I think there is a spectrum that full and semi-public falls on, where as hidden public is a specific type of activity.