Cylindricity is a form control, so it takes no datum. It bounds the entire cylindrical surface of a feature between two coaxial cylinders separated by the tolerance value, which means it simultaneously limits how out of round the feature is, how bowed its axis is, and how much it tapers along its length. That combination is what makes it powerful and what makes it expensive.
What the tolerance zone looks like
Picture two perfect cylinders sharing an axis, one just inside the other, with the radial gap between them equal to the tolerance. Every point on the real surface must fall in that gap. Nothing in the callout says where the axis is or which way it points, because form controls only ever compare a feature to itself. That is why cylindricity never carries a datum letter.
Why it costs more than circularity
Circularity is checked in one cross section at a time and a shop can do it with a bench centre and an indicator. Cylindricity has to be evaluated over the whole surface at once, which in practice means a coordinate measuring machine or a dedicated roundness instrument taking many traces and fitting them together. Calling it out where circularity would serve turns a five minute check into a metrology job.
When you actually need it
Use it where the whole length of the surface does work: a hydraulic bore that must seal along its travel, a bearing journal, a plug gauge, a pin that has to slide without binding over its full engagement. If only one plane of the feature carries load, circularity in that plane is the honest and cheaper control.
How shops hold it
Turning alone will get you close on a rigid part in a good machine. Holding a tight value usually means finishing by grinding or honing after turning, with the part supported so it does not deflect, and letting it come to temperature before the final pass. Every one of those steps is priced, so the value on the drawing decides which of them the shop has to buy.
Questions people ask about cylindricity
Does cylindricity need a datum?
No. It is a form control, so it compares the feature only with itself. Any callout that shows a datum reference beside a cylindricity symbol has been drawn incorrectly.
Is cylindricity the same as roundness?
No. Roundness, called circularity in the standard, applies to a single cross section. Cylindricity applies to the whole surface and therefore also limits straightness of the axis and taper along the length.
How is it measured?
By taking many circular traces along the feature and fitting two coaxial reference cylinders to the whole cloud of points. A coordinate measuring machine or a roundness tester does this; a micrometer cannot.
What value is reasonable?
It depends entirely on function, but it must be tighter than the size tolerance of the feature to mean anything, and every step tighter tends to add a finishing operation. Ask the shop what it can hold before fixing the number.