File-permission mapping for the NTFS file system is required because the Windows security subsystem provides a more robust means of access protection than is available under UNIX file systems. Under UNIX, a file has a single owner and a single group, with certain permissions (read, write, and/or execute) granted to each. Separate permissions are also granted for others, meaning any other users who are not members of the group that owns the file.
NTFS, on the other hand, allows permissions to be assigned to many users or groups independently. For example, an NTFS file can specify that User A is to have read-and-write access, while User B is to have only read access. The same file can also have certain permissions for one or more groups. The objective of Server for NFS is to provide a consistent, intuitive mapping of file permissions that UNIX clients can work with, and users can understand.
For information about how Server for NFS maps NTFS permissions and UNIX modes to each other, see Understanding Windows and UNIX file system security.