column grid label with apostrophe in technical drawings

structure scholar right here! i cant appear to seek out data on why some columns are labelled the identical letter/quantity simply with apostrophe. for instance, within the pic there's a column 8 and eight'. can somebody inform me the that means of it and when is the apostrophe applicable to make use of? thanks a lot :")

https://preview.redd.it/ozmpd73j54lc1.jpg?width=1414&format=pjpg&auto=webp&s=f2cc08a864a63bd507071509156942dd8fe10ea9