Answer:  The ANSI standard for ACME screw threads provides for two classes of fit, “General Purpose (G)” and “Centralizing (C)”.  The General Purpose fit provides for clearance on the major diameter and minor diameter while the screw and nut interface takes place on the thread flanks.  The Centralizing fit provides for the same flank contact while simultaneously contacting on the major diameters.  For applications of purely axial loading, there is little difference between the two.  Centralizing fits are often preferred for applications that have side loading since the major diameters act as radial bearings to maintain concentricity.