I believe it means “True Point Indicated.” It is not referring to a “position.” It refers to a “point” (or ring – around the part) where the shunt angle meets with the step. We don’t specify the diameter with a tolerance at that point because of the difficulty in measuring that feature. For that reason, we specify the diameter with tolerance at a gage point slightly above, for better & consistent measurement ability. The TPI is called out as reference, just so that it is clear what the theoretical diameter is at that point