You fly into one country, travel overland for a month, and fly home from somewhere else. It's a great itinerary. It can also produce a surprisingly awkward check-in conversation: Where is your ticket out of the country you're entering?
You show your flight home. The agent points out that it departs from a different country. Suddenly a trip that made perfect sense on your map looks incomplete on a booking screen.
The fix is to identify the missing border-crossing leg. For proof of onward travel, the relevant question is often how you leave the first destination, not whether you eventually have a flight home.
An open-jaw trip has three separate pieces
Travelers often call this an open-jaw itinerary: you arrive in one city or country and depart from another. The journey can involve a bus, train, ferry or additional flight between the two.