Payload

In cargo transport, payload is the valuable contents of the vehicle.

Similarly, in military aircraft, payload is the carrying capacity of an aircraft, whether as cargo, munitions, or external fuel, although internal fuel is usually not included.

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In communications and information science, a payload is a set of data, such as a data field, block, or stream, being processed or transported, the part that represents user information and user overhead information, and may include user-requested additional information, such as network management and accounting information.

Note: The payload does not include system overhead information for the processing or transportation system. Synonym mission bit stream.

Source: from Federal Standard 1037C