This entity is not in the game by default, but got left over in the FGD.
ai_addon_builder is a point entity available in all Source games since Alien Swarm. This entity was part of the "Directed Design Experiments" performed at Valve in November 2007, and is the base of a modular AI system, wherein enemies could collect upgrade chips to gain new abilities and drop them when dead, allowing other enemies to collect and reuse them. This code has never been released to the public or used in its entirety in a Valve title.
A related keyvalue called
addon was defined in
CBaseEntity, and a related input called
CreateAddon was defined in