It's a taper tolerance. In the past, there were some for turned pieces (turned pieces) with a small diameter and long when you don't work between points.
If you don't finish with a minimum removal and at a high speed for the last pass, then the tool flexes the part a little at the end and this generates a certain taper. Typically when you make rough drafts in rough mode or with poorly sharpened tools. With the carbureted pads this problem has almost disappeared. Note that the GPS rating does not include this taper problem because it is treated differently.
Be careful not to confuse it with the taper symbol used for walrus cones or tapered pins.