The Traffic Situational Display is a depiction of the entire US airspace, and is the primary source for determining the status of the NAS in the ATCSCC and other levels of traffic management above the level of ARTCC.
The important thing to understand here is the basic way in which traffic management operates.
FAA radar sites are connected only to ATC centres in their vicinity. They do not connect to every site, and no one can see the feed from every radar. This is fine in local sectors, as they only want to see their area, but it doesn't work for those wanting to manage the entire system.
Thus a secondary system is implemented, that displays the entire NAS, but is not fed from radar. Instead, it displays flight plans. When an aircraft takes off, a flight plan is filed into the TSD, and anyone watching the TSD can follow the progress of said flight. When the flight plan changes, the ATC site handling that flight can modify the flight plan in the TSD, so that it remains up to date.
The problem with 9/11 is that people started using the TSD to track flights. When AA11 turned south, deviating from its flight plan, NY ARTCC entered a second flight plan in the TSD; AA11A. However, the original flight plan for AA11 was never removed from the system. In fact, AA11 continued on the TSD, merrily following its original flight plan west. Twice, the ATCSCC called ARTCCs asking them if they had the aircraft on their scopes, after the original TSD track entered their airspace. The AA11 track would eventually continue on to land in Los Angeles, just as scheduled. Of course, while AA11 flew to Los Angeles on the TSD, and AA11A continued flying south, the actual aircraft had already crashed.
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