From YPPedia

An archipelago [ˌɑːkɪˈpɛləgəʊ] is a collection of islands within an ocean. Its boundaries can be seen by the longer routes that connect archipelagos together and by chart colors. Each archipelago is associated with a color. Charts to islands in that archipelago (intra-archipelago) are marked with its color, and inter-archipelago charts have the colors of the territories at each end. The articles on islands and archipelagos also have mini maps that show each route in a different color giving an indication on the difficulty of the route.

Once an island has one or more functional buildings (construction sites or others), a ferry service is instated that allows transportation to the other islands within the archipelago with functional buildings.

A separate Ultimate list is maintained for each archipelago.