The tolerance zone is defined by two parallel planes above and below the best fit plane through the measured data. The tolerance planes are separated by the tolerance t. As with the line above, if we have measured 20 points. The best fit plane is the plane where the sum of the errors (hills and valleys) is a minimum.
Don’t forget: The flatness of a plane makes no comment about how level (wrt gravity) a plane is.
The Tolerance Zone is here, like flatness, defined by two parallel planes above and below the data. The difference here is that the planes must be orthogonal (at right angles) to gravity. In plain English: These measured points are described as level or “in water” when they lie within a given tolerance (t) to a supposed waterline drawn through the average height of the points.