The core technology in SVS is known inside Symantec as "Fortress". The Fortress file system filter driver -- fslx.sys -- is the traffic cop that controls interaction between the operating system and applications in SVS virtual layers.

In the interest of simplicity, Fortress is architected to leverage and extend native Windows functionality wherever possible. This approach ensures that virtualizing an application has the minimal possible impact on the appearance, behavior and supportability of that app. For example, fslx.sys maintains the barrier between the real and the virtual by tracking Windows process trees -- keeping things in order the same way that windows itself does.