This is a post by Cat McClintock on PTC site.

In general, when engineers create or modify a design, they do much more than show lines, holes, curves, or even solids. That’s because geometry alone can’t communicate everything needed to turn a design on paper (or on the screen) into a real-world object.

So, traditionally, engineers create a 2D drawing with notes (or annotations) showing dimensions, materials, tolerances, etc. Then, anyone who needs to make or assemble the parts can find all the information they need with the geometry. 

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