144 hours visa free transit in China

To be eligible for 144 hours visa free transit in China, it is also a requirement that travellers must have an onward air / vessel / train ticket to a third country (region) with confirmed date and seat within 144 hours upon arrival. I don't quite understand this requirement. For example: when we order an flight ticket we only receive our e-itinerary which shows flight date but no seat information and it does not contain the wording 'confirmed'. Moreover, we can only get our seat information 24 hours before while doing online check in. Does this kind of e-itinerary meet the requirement? Can somebody be kind enough to clarify it? Thank you.