Here are a few links describing mechanical dimension terminology and some rules.
The videos with examples are better than just a lot of text rules, IMHO.

https://lph.lpisd.org/apps/video/watch.jsp?v=97611

https://lph.lpisd.org/apps/video/watch.jsp?v=97676

http://www.me.umn.edu/courses/me2011/handouts/

http://www.me.umn.edu/courses/me2011/handouts/drawing/blanco-tutorial.html

http://www.pages.drexel.edu/~rcc34/Files/Teaching/MEM201%20L5-Fa0809-SpDimensions_RC.pdf

- Brian