A VSAM data set must be marked nonreusable before you can build an alternate index over it. This VSAM requirement applies whether or not DFM is used to build the index.

If DFM is used to build the index, it automatically changes the base ESDS or KSDS to nonreusable, before it creates the alternate index. It is also possible for an z/OS® user to mark a data set nonreusable with the IDCAMS ALTER command and the NOREUSE parameter.