Final answer:
The NDR flag stands for No data received, indicating that a network session has been initiated but payload data has not been transmitted.
Step-by-step explanation:
The NDR flag in the session table typically stands for No data received. When set, this flag indicates that the session has been initiated, but no payload data has been received. It's a state that could be used, for example, in network monitoring or firewall systems to track the status of network connections. If expected data is not received within a certain timeframe, the session might be considered inactive or broken, and thus this flag can be useful in diagnosing network issues or ensuring efficient resource management of network session tables.