The first level-one VIER is created with the VTOC index. Subsequent VIERs are created whenever a VIER is too full to allow a data set name to be added to the VTOC index. VIERs have the following characteristics:

• A VIER uses one VIR and contains variable-length index entries. The number of VIERs in an index depends upon the number of data sets on the volume.
• All index entries within a VIER are at the same index level. VIERs in a VTOC index can be on several levels and have a hierarchic relationship. Index entries in higher-level VIERs point to lower-level VIERs. Index entries in level-one VIERs (those at the lowest level) point to format-1 or format-8 DSCBs for data sets on the volume.
• Whenever a fourth VIER is created on the same level, a VIER at a higher level is created. Once a higher-level VIER is filled with pointers to lower-level VIERs, another VIER at the same level is created.