you dont have to go customs untill your destination but you *might have reclear TSA secuirty though
but on the return you will clear US customs in SFO

You will probably have to go through customs in Seoul and then again in Mongolia. And when you come back to the US, you will have to go through customs wherever your first stop is.

You only go through Customs when you enter or leave a country (many countries do not have Customs screening for departing passengers, and a few do not have Passport Control for departing passengers).

If your airline has an "International Check-In Desk" in Seattle then use it as your itinerary includes international flights. Don't spend time looking for the "International Desk" as most airlines in the USA no longer have separate check-in areas for international and domestic passengers.

Same with security - unlikely for there to be separate international and domestic check-in areas. However, your international itinerary MAY result in more scrutiny, so be at the airport two (2) full hours before your first flight.

You will check your bags to your destination (Mongola). Make sure the airline agent puts the right 3-letter codes for your trip on the bags for eaxample, San Francisco is SFO, Seoul (Inchon Airport) is ICN, and Ulan Bator (Chinggis Khaan Airport) is ULN.

You may or may not have to go through security when you change planes in SFO. The airline can tell you.

Assuming you are just changing planes in Seoul, you should not have to go through security or Passport Control & Customs there. Just follow the signs to the termainl/gate for your next flight. Ask an airline employees for directions/help if you are not certain what to do.

You may have to collect a boarding pass in Seoul from the airline you are flying to Mongolia. If so, just ask for directions from the airline staff that meet your SFO-ICN flight in Seoul.

At your destination,you normally exit the aircraft, go through Passport Control, collect your bags, and then go through Customs.

Good luck!
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