In the modern world where a 2D engineering drawing is required we create many section views along the length of the geometry. Think of a loaf of bread and each section view is where the slice is and that would be detailed. For this system to work there must be a common reference or zero location from which the dimensions and tolerances are established. This is called a datum. Additionally, there would be an engineering note on the drawing that would specify the dimensional conformance between the section views (slice point) indicating the dimension and tolerance conformance requirements.
Where a 2D engineering drawing is not required we use 3D model based definition methods. This is where the geometry is manufactured directly from the CAD model. We have technology that can read the 3D model and manufacture and dimensional tolerance verify.