Will Onward ticket without seat assignment be acceptable for 144 hours visa free permit

To apply for the 144 hours free transit, I need to present valid passport, visa for a third country or region if required, a full completed Arrival and Departure Card and an onward ticket with confirmed seat to the staff at the Transit Service Counter.
My onward ticket is a confirmed e-ticket without a SEAT Assignment, is that acceptable?
My route is Mongolia to Beijing, Beijing to USA with 3 hours layover in Seoul. When I fly in to Beijing from Mongolia, I only have the e-ticket confirmation but no seat assignment.