What is the catch-all zone for?

It catches every country that none of your other zones lists. So every parcel always has a zone and a price. There is exactly one catch-all zone. It has no list of countries and cannot be deleted. In a new store it is called Rest of world.

Orders that land in it are flagged, and that flag is useful. A country that keeps landing there is either a market worth its own price, or one you may stop shipping to. Both are easier to decide from a flag than from a surprising carrier bill.

Give the catch-all zone a price on your rate card too. In a new store it is the World column. A zone with no price is flagged as unpriced, never treated as free shipping. To see which countries land there now, click its column header.