Tolerance is equal to algebraic difference between upper and lower deviations and has an absolute value without sign. In the context of this terminology for limits and fits, the difference between maximum limit of size and minimum limit of size is called tolerance.
In the figure given below, tolerance etc. are shown too much exaggerated. It is assured that shaft touches at the bottom of hole and then other dimensions are considered. A line is assumed to be there in between upper tolerance on shaft and lower tolerance on hole. This line is called zero line.
This also refers to basic size from axis of the shaft or hole. All tolerances and deviations are referred to from this datum line. Setting of tolerance value alone is not sufficient to define particular limits, but position of the tolerance zone relative to the basic size of the feature must also be specified.