The Letter before the first slash is the tech level rating. It's a rating of how advanced an industry is required to produce the item.
The three letters between the slashes are the availability during three different eras: The Age of War through the Star league, the Succession Wars, and the Clan Invasion area and after.
The letter after the last slash is the legality rating.
The lower the letter, the easier it is to get a hold of, for the availability ratings; An A means the item is ubiquitous, D would be a somewhat rare item, and F is a unique or collectors item; an X means it doesn't exist in a time period.
Everything is detailed in AToW pg. 254-255
