What a collection is
A finite set of places with a total attached. A country, a region, a category such as waterfalls or temples, or a list somebody sat down and made. The total is what turns a map into something you can finish, and it is why a collection page shows the places still ahead of you on the same screen as the ones behind.
Countries, regions and categories work out their own membership from the places themselves, so a new place in Japan joins the Japan collection the day it is added. That cuts both ways, and it is worth knowing before you go looking for a bug: a collection you have finished can open back up when the guide grows into it. Progress here follows the catalog rather than freezing the moment you close something out.
