GD&T Cylinder Parallelism

Cylinder parallelism is the maximum value of the measurement of the taper of the cylinder.

It is calculated as the parallelism of two least squares lines constructed through the vertical sides of the profile.

Reference: https://www.taylor-hobson.com/resource-center/faq/what-is-cylinder-parallelism

Advanced Tubular Note:  GD&T profile of a cylinder is similar but not quite the same - because the profile value gives the maximum distance of the high and low of a surface relative to its nominal.