when measuring, all values are uncertain
ex. machine that calculates to nearest 0.0001 grams, measurement is 2.6007, but it is not certain if real measurement was 2.60068, 2.60071, 2.600715686, etc., and if the machine rounded the last digit up or down, so you would write it as 2.6007 +/- 0.0001
\-since it is agreed that all values are uncertain, measurements are usually reported without the +/- as it is assumed that it is uncertain.
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\-any measured value with digits