A recent question regarding allowable elements of a heraldic achievement sent me running for the best reference I know: a paper by Andreas von Meißen presented at KWHSS in 2013.
This document is admittedly out of date — among other things, it predates Avacal, and it does not include several changes that were made to the rules in Ansteorra the following year, based on Andreas’s recommendations — but I’m not aware of anything more recent or more authoritative, and so this remains a useful guide.
For more information, and loads of useful context about historical practice, review the original paper Achievements of Arms – A Historical and SCA Perspective at Andreas’s Paly-Bendy site.