With CallbackDisk you can create and manage virtual disk devices within your application and expose these disks to Windows as if they were real disk devices. The disks become visible to all process in the system.
Virtual disk is the one that has a file system on it, but is not physically located on a disk partition. The storage can be actually located in a file residing on a local disk, or in applicaiton resources, or in memory, or somewhere at a remote location. The important part is that this disk is visible to the user (and to other applications) as if it were a formatted hard disk partition.
CallbackDisk provides a kernel-mode driver, which calls your application when it needs to perform reading or writing to the disk. And your application can store the actual disk data anywhere it needs: in memory, over the net etc.
Virtual disk can be needed, when
* the application stores some data internally (in resources or in supplementary files) and it needs to use some library, which can only work with file located within a system-wide file system. The examples include DirectX libraries, Flash player, many third-party components.
* the application must hide presence of the disk from the user, while keeping it (the disk) available for some applications
* and more ... | 
