Not long ago, I wrote a short post about non-standard use of reference designators. After doing that, I've been looking at some of my own microcontroller and motor driver boards with an eye for how close to standards I am.
All of the R's, C's, D's and U's are okay, but there are some differences. For example, the Eagle library I've been using calls crystals "X" instead of the more standard "Y." I have seen crystals designated as "X", "Y" and "Q." LEDs seem to go by "LED" instead of "D" as indicated in the Wikipedia list. Headers go by "J", "JP", or "H." Wikipedia says "J" is for a female jack connector, "JP" is for jumper, and it doesn't list a "H." My board has break away two-row male headers and keyed single-row male headers. Wikipedia does note that its list is a set of commonly used designators. Not necessarily standard.
We probably do have the specific standards document laying around here someplace, and if I were doing real work on a professional basis, I'd hunt it down and make sure I followed the actual standards. But I'm not doing real work with my controllers and drivers, so I just do the best I can. I wonder how often that happens everywhere. The standards books are "somewhere" but no one really knows where.
Somwhere over the reflow...