Parallels - what are the differences?

[f350ca]

I use HSS tool blanks quite often. They're ground perfectly straight and parallel, and I've managed to collect a good assortment of sizes from 1/8 to 1 inch.
On the bearing race, I have a large one, about 5 inches in dia I use to tram the mill. Again they're ground to closer specs than I can measure, lay it on the table and set the dial indicator to sweep it from the chuck. Full measure in one setup.