It means a numerical value commonly used to indicate the coaxiality/concentricity between one shaft and another. The place where two circles intersect and overlap is called a double circle.

If two circles are separated and have no overlapping parts, that is, they have no common elements, this is also a relationship. In addition, there may be some overlap between the two circles. That is, some elements belong to both set 1 and set 2, but set 1 and set 2 also have some unique elements.