[This is preliminary documentation and subject to change.]
On an NTFS version 5.0 volume, a file or directory can contain a reparse point, which is a collection of user-defined data. The format of this data is understood by the application which stores the data, and a file system filter, which you install to interpret the data and process the file. When an application sets a reparse point, it stores this data, plus a reparse tag, which uniquely identifies the data it is storing. When the file system opens a file with a reparse point, it attempts to find the file system filter associated with the data format identified by the reparse tag. If such a file system filter is found, the filter processes the file as directed by the reparse data. If no such file system filter is found, the file open operation fails.
For example, in Windows NT 5.0, reparse points are used to implement NTFS links and the Microsoft Hierarchical Storage Management (HSM) Server. HSM Server uses an administrator-defined set of rules to move infrequently used files to long term storage, such as tape or CD-ROM. It uses reparse points to store information about the file in the file system. This information is stored in a stub file that contains a reparse point whose data points to the device where the actual file is now located. The file system filter can use this information to retrieve the file.
The following topics discuss reparse points: