On VMS machines, indexed files are implemented using RMS indexed files. On MS-DOS, UNIX, and MPE/iX machines, the Vision indexed file system is used by default. The Vision file system is described here.
Vision files may be created with either single or dual file format. With Vision Version 3, files are generated in a single file that contains both the data records and the overhead key information. A separate linked list is maintained which is used to rebuild corrupted files. A linked list of deleted records is also maintained so that new records first occupy old deleted records before expanding the file.
In Vision Version 4, each file occupies at least two disk files--one file, or segment, contains the data records and another segment contains the key information. When any segment approaches the configurable file size limit (up to nearly 2 GB), Vision creates a new data or index file, in which it stores the excess information. Separating data files from index files also provides a high degree of reliability when files must be rebuilt. Permanent data loss due to memory-related problems is less likely to happen to two files as opposed to one file.
More:
Segment Naming for Vision Version 4 Files
Method Two: The Default Method