Look up ASME Y14.5. It's an international standard offering guidancefor dimensioning designs andapplying tolerances to those dimensions (giving manufacturers an idea of how much variance you are willing to accept).It is generally referred to as GD&T, or Geometric Dimensioning and Tolerancing.
Datums are used to provide a reference from which dimensions are generated. Datums can be planes, axes, and even features. Renaming the datums to A, B, and C is not actually necessary. Per the guidance, you can name the datums anything you want. The guidance uses A, B, and C by default for primary, secondary, and tertiary references, simply because it's easier and makes the resulting feature control frames more manageable. The feature control frames could get messy and exceedingly large if you use longer names.
Bear in mind thatin general you do not have to use GD&T as discussed in the standard. Some companies require it, some may even lay out specific guidance on how to use it or add to it(companies like GM and GE among many others helped develop the standard over many years). In the end what really matters is that the people interpretting your drawing/model understand it, and when they make the item from that drawing/model, it turns out how you wanted.
I've taken classes on GD&T and I think it is very useful for designers, even as more machinists work directly from 3D models becauseI think it helps designers plantheir designs for ease of manufacture. However, it can take a while to get used to it. I would not suggest trying to learn through usingPro/E, but rather in a classrooom setting with a teacher who has usedGD&T in industry.See if your company can offer a class for all of your designers. I know GM used to provide yearly refresher courses for their engineers.
