A coloured illustration on page 8 of Sword Art Online Volume 07.
Latest image source: Ruranobe
Original image source: Gsimenas
Sword Art Online Volume 07, Yen Press releaseEdit
Officially known as "Original Sword Skills" (OSS). For the various kinds of weapons that were implemented into ALfheim Online, this is a new system of attacking. It's an updated version of the Sword Skills from the old Sword Art Online.
The difference between OSS and the former SAO's Sword Skill tree is that, rather than basing everything on the ability to hit predetermined stances and motions, the player himself can create and register his own skills. However, because the system demands that players achieve their combos at physically impossible speeds without any system assistance (a paradoxically difficult task), only a handful of players have ever succeeded in creating and utilizing their own OSS.
However, the Original Sword Skill system has a feature called "skill inheritance". Those who have successfully created a skill can fashion a single-generation copy of that skill in form of a "Skill Tome", which allows another player to learn and use that OSS. Because creating an OSS is so difficult, these Skill Tomes are among the priciest and rarest of items in ALO - particularly the Tomes for combos that include over five hits.