Answer:
1 sf is the area represented by a square that has a length of 12 inches (1ft) and a width 12 inches (1ft) This area does NOT have to be a square, it can be any 2 dimensional shape.
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1 sf could also mean 1 significant figure. That is a value rounded in such a way that it contains only one non-zero digit. For example:
4321 = 4000
or
21.34 = 20
or
0.0063 = 0.006