Table 20

+=============-===============================================================+ 
|  Operation  |  Description                                                  | 
|  code type  |                                                               | 
|-------------+---------------------------------------------------------------| 
|      M      |  Mandatory - Commands so designated shall be implemented in   | 
|             |  order to meet the minimum requirement of this International  | 
|             |  Standard.                                                    | 
|             |                                                               | 
|      O      |  Optional - Commands so designated, if implemented, shall be  | 
|             |  implemented as defined in this International Standard.       | 
|             |                                                               | 
|      V      |  Vendor-specific - Operation codes so designated are available| 
|             |  for vendor defined commands.  See the vendor specifications  | 
|             |  where compatibility is desired.                              | 
|             |                                                               | 
|      R      |  Reserved - Operation codes so designated shall not be used.  | 
|             |  They are reserved for future extensions to this              | 
|             |  International Standard.                                      | 
+=============================================================================+