(a–zc) are used for external features ( shafts ).
Curious, Alex downloaded the PDF and began to explore its contents. The document outlined the international standard for tolerances on linear sizes, providing a framework for engineers like himself to ensure interchangeability of parts. iso 286 pdf
For example, if you have a shaft with a nominal diameter of 25mm and a tolerance class of k6 , you need the PDF (or a software tool based on it) to determine the upper and lower deviation limits. (a–zc) are used for external features ( shafts )