Traditionally, when you book tickets through airline's official website, you might book all segments of flights in the same ticket, so it will be possible to check your baggages to final destination from initial departure, and if you missed your connection flight you might be arranged for the next flights. These tickets are called "Transfer guaranteed" tickets, the vertical green line next to the airport names is connected.
"Transfer not guaranteed" means those tickets are sold separately, and you have to book tickets for each of segment of the travel. Upon arrival you might have to claim luggages, clear the customs and immigration(for international flights), and re-checkin for next flight. For each segment of the entire flight, you might need multiple visa or local immigration documents before entering the country. If your last flight was delayed too long that check-in counter or gate for next flight was closed, you might be considered as "no-show." Such flights are indicated with a separate vertical green line.
However, direct transfer services ("Transfer guaranteed") are only provided by some airlines for some flights in some specific stops (e.g., Scoot's Scoot-Thru, and AirAsia's FLY-THRU). There are also some airlines do not provide transfer services at all, even if all flights are issued on the same ticket. All Smart Combinations we provided are partially or wholly "Transfer not guaranteed" tickets, you can turn this option on/off in Filter, or check the green line.
We suggest you check Airline's website, or local authority like DOT for your rights and regulations before booking tickets, and if you're booking "Transfer not guaranteed" tickets, please reserve 2~4 hours between arrival and next take-off and get your travel insurance covered.