A reparse point is an object in a file system with attributes that activate extended functionality. A tag in the reparse point indicates the location from which external information should be taken and specifies an application associated with that information.
A single file can contain more than one reparse point, with each point involving a different application. When a system opens a file and encounters a reparse point, the system finds the filter associated with the application indicated in the tag. The data in the reparse point can then be used to transparently execute whatever task is specified, through the application that created the reparse point.