The organization reveals this information on their site. They discuss a system of sending and receiving messages to and from airplanes in flight called ACARS, or Aircraft Communications Addressing and Reporting System. They have quoted a United dispatcher named Ed Ballinger as saying that the second time stamp shown on the messages is the acknowledgement of receipt, a key point because the acknowledgement proves the flight had not crashed at the time of the last known message, 9:23 AM (the alleged crash was at 9:03 AM).