Full width exposure of track area of sound negative. 1
Flood track is produced by a sound camera. The sound negative density is read using a densitometer on a full width flood track.
https://www.nfsa.gov.au/sites/default/files/uploads/2010/10/13/floodtrack.jpg
References
1 Case, D., 1985, Motion Picture Film Processing, Media Manuals, Focal Press.