Unused Datum? Base Alignment?

[[Ja...]]

I recently had a print that specified a surface as C, but nothing was being called back or referenced to it. I was informed this surface was intended to block the rotation (planar) of the part in the alignment. That being said, it was my understanding that your base alignment could be derived from whatever features you saw fit (notably the most repeatable features); and that the datums served only as references for measurements. Am I correct in that assumption? Or do Datums = Base Alignment?

[[Br...]]

Datums do nothing if they aren't referenced.

https://youtu.be/S8eh4yxfEdk

[[Ro...]]

life is hard enough without making up extra constraints.

[[Ja...]]

Gentlemen, thank you both! I had thought I was on the right track. I've only been in the game a couple years now, and this forum is a wealth of knowledge.