property UniqueID: string;

UniqueID is a string property that uniquely identifies a file or a directory on the remote file system for the FTP server. UniqueID is useful for file systems that allows symbolic links where multiple links could actually resolve to a single file. 

The value in UniqueID is assigned when a directory listing parser encounters the "unique" fact in a directory listing response. When the "unique" fact is not present in the Facts for the collection item, the value of UniqueID is an empty string ('').