Ephemerisle is not an event. It's a distinct collection of independent groups who show up during the same week.
An "island" at Ephemerisle is an independent and autonomous collection of one or more anchored boats, platforms, and/or whatever else floats that's the home of at least one person during the gathering. Generally they have a theme. In the past, Islands have been encouraged to experiment with their own governance systems. Every island is strongly encouraged to treat it's neighbors with respect and not doing anything (including anchoring too closely) which might put their neighbors in danger.
An "Archipelago" is an extension of the idea of an Island where different groups of boats join together into a larger collective. Like Islands an Archipelago can have any form of government they chose from a strict hierarchical dictatorship, to a very loose confederation.
"Archipelago Islands" are a subset of boats within an Archipelago, and they differ from independent Islands as they are not fully independent or autonomous.