The Cardinal System (カーディナル・システム), or simply Cardinal is the game engine of nearly all VRMMORPG games in the Sword Art Online series. (A game engine manages all resources, processes, and events of the game, and can be simply defined as the works of a game's world.) Kayaba Akihiko designed the original system. He had made more beginner and modder-friendly so it could be easier spread in the form of the World Seed that Akihiko eventually gave to Kirito.

Game Management

The multi-threaded and incredibly profound Cardinal System was programmed to manage the entirety of Sword Art Online, including real-time rebalancing of items and currency, keeping check on the players' mental state, changing the world as needed, and generating new quests and even new non-playing characters. The only other Virtual Reality Massive Multiplayer Online game that had the same level of complexity was ALfheim Online, but the resource-hungry quest and AI generation routines were disabled until the game control was transferred to Ymir.

Quest Creation

The full Cardinal System is able to create new quests based on the stories that it finds online, which of course led to multiple immersion-breaking quests in SAO. It would also sometimes restructure the game world for everyone following up to the more important quests completed by some players. As one example, in the second season of the TV anime, the Excaliber[sic] quest resulted in the World Tree destruction in ALO, accompanied by the loss of player-owned homes and items.

Gameplay Management

Any version of the Cardinal System's autobalancing functionality includes means of solving the balancing issues that work as a separate system process that constantly checks for the leftover by-design issues. A player who found an easy pack of enemies with solid XP boost would find their XP adjusted later; a system glitch would be patched, or the administrator summoned in non-trivial cases. It would even generate new AI-based characters (like Freyja and her sisters in the Excaliber quest) who would be complex enough to react like a normal person to unexpected input — for example, Sculd, a non-talking NPC, waved back and sent divine sparkles in response to Klein's sudden love confession. The friendly AI Yui who came to be Kirito and Asuna characters' daughter also had been generated by the Cardinal in SAO.

Monitoring

The Cardinal System's monitoring processes are run in parallel for bug detection and keeping track on the players' mental state.

Emotional Monitoring

The original NerveGear could analyse the player's brainwaves to gather data on their emotional state. An AI process Yui-MHCP001 was created by Cardinal of the beta-stage SAO to make use of the data and possibly help fix any issues that would arise. For the official launch, however, Kayaba Akihiko had changed Yui's permissions so she could not interact with the players and was left to mere monitoring and analysis.

Cardinal also could sometimes use an unusual mental pattern to tweak the landscape, items, other players' avatars for analysis purposes. As an example of this, when Asuna and Kirito solved the murder mystery saw the ghost of the victim Griselda smiling at them by her grave for a few seconds, which is surmised to have been this effect "echoing" from her wedding ring and the landscape.

Alicization

In the Alicization Arc, the Underworld flavor of the Cardinal System had been used. During the activity of the Axiom Church organization, its administrative control was mostly overridden by the Church's founder Quinella who had flashed the main bulk of Cardinal into her Fluctlight on accident. Following this, Quinella developed a split personality, whereupon the error monitoring and correction set of processes had emerged as a semi-independent side of her consciousness. The error control persona decided Quinella's balancing control was lopsided and in need of correction. Later on, the persona got a body for itself.

References

Cardinal System - Fan Wiki