On Dec. 2, the US Patent and Trademark Office issued US patent #7,461,096, "Weighted prioritizing layered computing system." This is the fourth patent issued around the Fortress layering technology acquired with FSLogic by Altiris in early 2004. Fortress is the basis for Symantec's industry-leading application virtualization product, SVS. The full text and diagrams for the patent may be read here.
As has been discussed in other Juice articles, the functionality in SVS is made possible by a concept called "layering". Think of a layer as an overhead transparency. The transparency has file system and registry objects on it. When several of these layers are stacked on a projector, the screen shows you the aggregate of all the layers/transparencies. The specific behavior of software running from one of these layers -- and how other software on a system interacts with the layers -- is governed by the fact that the layers are in a predictable order. This patent describes how and why layers can be prioritized in order to yield different desired behaviors.
Patents, of course, can be somewhat difficult reading, even for an engineer. And if you do read it, keep in mind that the actual implementations of patented concepts in shipping products may differ slightly from the examples in the patents. So this is not SVS product documentation. However, for the hardcore SVS fan (or competitive analyst), reading this patent will give you deep insight into what was going through our minds as we improved the design of SVS 2.0 during the original year-long beta in 2005.
For some more plain-speak discussion of some aspects of these concepts, as they have been implemented in the actual SVS product, see Launching Two Versions of an App When Both Are Installed to the Same Location and Inside SVS :: How (and Why) fslx.sys Tracks Windows Processes.