Form Tolerances: Flatness, Straightness, Circularity, Cylindricity
Form controls never use datums
Form tolerances constrain a feature to itself, so the four form controls never reference datums. Flatness places a surface between two parallel planes. Straightness does the same for line elements, or for the derived median line when applied to a feature of size. Circularity (roundness) limits each circular cross-section individually between two concentric circles. Cylindricity limits the whole surface between two coaxial cylinders and therefore controls circularity, straightness of elements, and taper simultaneously.
Two traps dominate. First, circularity is a per-section control: a tapered or barrel-shaped pin can be perfectly circular at every slice and still fail cylindricity or straightness. If the stem of the question mentions taper, barrel, or hourglass, the answer is cylindricity, not circularity. Second, straightness applied to a feature of size at MMC is the one control that can override Rule #1: a pin toleranced 10.0 to 10.2 with straightness 0.05 at MMC may bow, because the perfect-form envelope requirement is relaxed by that callout. It also earns bonus tolerance as the feature departs from MMC.
Remember that implicit form control exists everywhere: Rule #1 limits form at MMC, and every orientation, location, and profile tolerance limits form within its own zone.
Worked example: a pin toleranced 10.0 to 10.2 with straightness 0.05 at MMC. Virtual condition is 10.0 + 0.05 = 10.25 (external feature: MMC plus geometric tolerance). Produced at 10.15, the allowed straightness zone is 0.05 + 0.15 bonus = 0.20.