The quantity datatype allows to enter quantities. It has quantity value, lower, upper, and a unit.
NOTE: Quantities are assumed to be uncertain per default, exact quantities are considered to be an exception. The idea is that quantities are usually measures. Since this leads to confusion and discussion, here's the rationale for reference:
When doing arithmetics with quantities, such as unit conversion, defaulting to "exact" values would lead to "false precision" in the output.
From https://en.wikipedia.org/wiki/False_precision:
"In science and engineering, convention dictates that unless a margin of error is explicitly stated, the number of significant figures used in the presentation of data should be limited to what is warranted by the precision of those data."
And from https://en.wikipedia.org/wiki/Significance_arithmetic:
"If a calculation is done without analysis of the uncertainty involved, a result that is written with too many significant figures can be taken to imply a higher precision than is known".