The carried content, as against the carrying. The part that matters once the envelope is discarded.
From pay (Latin pacare, to settle) and load (Old English hlæstan, to lade); a 19th-century shipping term for the part of a cargo that earns its keep, later the part of any transmission that matters once the carrying is discarded.
The content actually being carried, as against whatever is carrying it — the part that still matters once the envelope is thrown away.