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The measurement of guitar string tension is expressed in units of force, usually in pounds (lbs) or kilograms (kg). The number 0.21 you mention could be an indication of the tension of the guitar string, but it is difficult to know for sure without more information.
In general, folk guitar strings are graded by thickness (diameter in millimeters) and tension. Thicker strings tend to have a higher tension than thinner strings.
If you only have the string tension measurement, it doesn't give you enough information to know which string you bought. It is likely that this measurement is a subset of a more complete string designation, such as "medium gauge folk guitar string drawing 0.21 lbs."
In short, the number 0.21 could be a measurement of the tension of a guitar string, but without more information, it's hard to know exactly what guitar string you bought.