Backgammon move notation - Is there an official, rigorous specification?
Paul Magriel's notation system originally appeared in his 1976 book Backgammon, and is mostly just a codification of notations systems that existed at the time.
This could be considered the "official" specification for the notation, but really it's usage is just customary.

His followup An Introduction to Backgammon also includes the notation, but has an easier to follow explanation as it is targeted at beginners.