No datum, ever
Flatness asks how far a surface departs from any perfect plane, straightness from any perfect line, circularity from any perfect circle. The comparison geometry is fitted to the feature rather than fixed to the part, so a datum reference has nothing to do.
A datum on a form frame is always an error, and it usually means an orientation control was intended: perpendicularity or parallelism, which do need a datum and also limit form.
What each zone looks like
A flatness zone is two parallel planes separated by the tolerance, free to be tipped to the best fit orientation. A circularity zone is a radial band between two concentric circles, applied section by section. Because that band is radial while size is diametral, a circularity value is a tighter statement than the same number applied to size.
Cylindricity uses two concentric cylinders over the whole surface, which catches out of roundness, bow and taper together. It is the most complete of the four and the most expensive to hold.
Straightness has two meanings
Attached to the surface with a leader, straightness controls individual line elements in each longitudinal plane and cannot exceed the envelope requirement. A shaft can be straight in every line element and still be bowed as a whole.
Attached to the size dimension with a diameter symbol, it controls the derived median line inside a cylindrical zone, and it is the common way a feature is permitted to violate the perfect form boundary at MMC. That produces a virtual condition larger than MMC, which has to be checked against the mating part.
When to add one
The test is whether the required form is tighter than what the size tolerance or an orientation tolerance already implies. A flatness callout looser than the parallelism tolerance on the same face changes nothing and only clutters the drawing.
Two cases almost always earn a callout: a sealing or bolting face, and a datum feature, because a datum feature with poor form gives an unstable datum and every measurement taken from it inherits the error.
Form permitted with no callout at all
A hole toleranced diameter 20.00 to 20.10 mm carries no form control. The envelope requirement still limits it.
- Perfect form boundary
- 20.000 mm
- Produced size
- 20.060 mm
- Form error permitted at that size
- 0.060 mm
- Form error permitted at MMC exactly
- 0.000 mm
- Form error permitted at LMC
- 0.100 mm
An explicit flatness or cylindricity callout on this feature is only worth adding if it is tighter than the row that matches the produced size. Otherwise it is a requirement that can never bind.