Tolerance is used to report errors on the Recipe Display. If the difference between the Preset Value and the Tags current value is greater than the Tolerance, an error is reported (a red e at the bottom left of the status bar) and the Preset Value is in RED and the Tag value is White.
Tolerance is in % of full scale (SpanHi - SpanLo). A zero value means that the tag must match the preset exactly.
Tolerance is used to account for the variation in the representation of numbers in the field device (e.g. 12-bit devices), the accuracy of IO and the conversion to/from engineering units.