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