It looks like it means Day-Second-Feet, which is a variation on Second-Foot Day (SFD).
Second-feet is a unit of measurement of liquid flow equal to one cubic foot per second.
A Second-Foot Day would be the volume of water represented by a flow of 1 cubic foot per second for 24 hours.
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Actually, DSF is acronym that stands for different things, so I cannot know to which you are referring to. But, just in case, if you go to any search engine and type "dsf" you will most likely get results like: Dubai Shopping Festival, Deutsches Sport-Fernsehen (according to Wikipedia, a privately owned German channel that features various things for different tastes), Desjardins Sécurité financière, and David Suzuki Foundation (Canadian environmental organization).
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$2.19
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