z/OS uses a Catalog and a Volume Table of Contents (VTOC) on each DASD to manage the storage and placement of data sets
The VTOC lists the data sets that reside on its volume, along with information about the location and size of each data set, and other data set attributes.
A standard z/OS utility program, ICKDSF, is used to create the label and VTOC. When a disk volume is initialized with ICKDSF, the owner can specify the location and size of the VTOC. The size can be quite variable, ranging from a few tracks to perhaps 100 tracks, depending on the expected use of the volume. More data sets on the disk volume require more space in the VTOC.
The VTOC also has entries for all the free space on the volume. Allocating space for a data set causes system routines to examine the free space records, update them, and create a new VTOC entry.
Data sets are always an integral number of tracks (or cylinders) and start at the beginning of a track (or cylinder).
You can also create a VTOC with an index. The VTOC index is actually a data set with the name SYS1.VTOCIX.volser, which has entries arranged alphabetically by data set name with pointers to the VTOC entries. It also has bitmaps of the free space on the volume.
A VTOC index allows the user to find the data set much faster.