Each market uses one shipping mode. Simple mode covers the whole market with free shipping, one flat rate, or a free-shipping threshold plus a below-threshold rate. Advanced mode uses shipping zones and merchant-defined methods.
Switching to advanced
Selecting advanced mode clears simple shipping amounts in the draft and adds one default Rest of world zone. The default zone is the catch-all for addresses no other zone claims. It cannot be deleted, unset, or assigned specific regions.
Switching back to simple
Switching back permanently removes every advanced zone, method, and rate. The dashboard tells you how many zones and methods will be removed and asks you to confirm before anything changes. The mode change and cleanup happen together: either all of it saves or none of it does.