From BrWiki
Advanced Dungeons & Dragons (AD&D) is regarded as the second version of Dungeons & Dragons. It too had a few versions; the last version being the second version. For this reason, it is commonly called Advanced Dungeons & Dragons 2nd Edition (AD&D 2ed or AD&D II). With the further development of D&D versions, it has also become known as D&D 2ed.
AD&D developed from the Basic D&D system in various ways, which include:
[edit] Advanced Dungeons & Dragons and Birthright
The original publications for the Birthright campaign setting set out the rules and NPCs according to the AD&D system. The BRCS is the development of those rules for other versions of D&D.