Am I eligible for the 72 hour policy??

USA issued passport. Departing 3/30 Chicago, IL > arrive 3/31 Shanghai Pu Dong, China (15 hour overnight layover) > departing to final destination 4/1 Seoul, S. Korea.

From what I've read, I understand that I will NOT need an entry permit and can take advantage of the 72 hour free transit. However, I get confused because under "Documents for Application" it states "Visa for a third country or region". What entry permit is this referring to? PRC, where my layover is or Korea, my final destination? I am not required to have an entry permit in Korea for the duration of my trip.
Thank you!