As you join curves, you are, in effect, chaining. You can pass the result to features as needed or to other curve and surface constructions.
The tolerance is used by Curve join in two ways. First, when the dialog decides how to order segments, it uses tolerance to decide if two segments are adjacent. If they are adjacent within tolerance, then they are considered to be an exact match and are joined together. If there is no match, then the closest segment is joined by using a straight line. Second, the tolerance is used when creating a final curve from all of the ordered segments. If the first point of the next object is the same as the last point of the previous object, the first point is not added. If the first point of the next object is not the same as the last point of the previous object, the first point is added to the new curve resulting in a line segment between the 2 points. When working with curves, FeatureCAM examines the last control point as compared to the first control point of the next. Arcs are converted to curves and manipulated as curves. When comparing points, FeatureCAM uses a Manhattan distance.