Volume reference

You can code VOL=REF on a DD statement to refer to a DD statement in the same or an earlier step to allocate the data set on the same volume as the earlier data set. You can also code VOL=REF to an existing data set (VOL=REF=A.B.C,...) where A.B.C is a cataloged data set. If VOL=REF is coded, SMS invokes ACS routines as follows:

SMS invokes the data class ACS routine.
Storage class is copied from the referenced data set if the referenced data set has a storage class assigned to it. However, if the referenced data set is an SMS-managed tape data set and it is not cataloged or DISP is not OLD or MOD, the storage class is not available. In that case, the ACS routine is called and must specify a valid storage class for the referencing data set to be an SMS-managed tape data set.
Note:
1. Non-SMS managed data sets do not have storage classes to copy. As a result, the storage class routine is invoked and either allows the non-SMS allocation or fails it. If it does anything else, SMS fails it.
2. When VOL=REF is coded to refer to a DD statement and the referenced DD allocates a new data set (DISP=NEW), the storage class is copied from the referenced DD if one is assigned.
  When VOL=REF is coded to refer to an SMS-managed tape data set, the storage class from the referenced data set will not be available. The storage class ACS routine is invoked and a valid storage class must be assigned; otherwise, SMS fails it. For example, in the case of VOL=REF to an SMS-managed tape data set, the storage class ACS routine must use the read-only variables &LIBNAME,&ANYVOL,&ALLVOL to derive a valid storage class that will allow this referencing data set to be allocated to SMS-managed tape. Additionally, read-only variable &LABEL can be used to help derive the valid storage class.
3. The storage class ACS routine is also invoked when VOL=REF and UNIT=AFF is coded on the DD statement. For information on what is passed to the storage class ACS routine, refer to Non-data set stacking allocations.
4. If you use VOL=REF processing to refer to a temporary data set, you might get different results in storage group assignments than expected. This is because temporary data sets are assigned a storage group by the system, based on a list of eligible storage groups, such as: VIO, PRIME, STANDARD, etc. Data sets that use VOL=REF are assigned a storage group based on this list of eligible storage groups, not on the name of the storage group used to successfully allocate the first data set being referenced. This might result in the data sets being allocated in different storage groups.
Management class ACS routine is called if storage class is not null.
The storage group from the referenced data set is passed as input to the storage group ACS routine. For data sets on SMS-managed tape volumes, the ACS routine must assign the same storage group to the referencing data set. For other SMS-managed data sets, any pool or VIO storage group can be assigned to the referencing data set.

When you specify VOL=REF in the JCL, the system retrieves the volume serial numbers from the referenced DD. In the case of NEW to NEW referencing, since DD is not allocated yet, the default of 1 is passed in the &NVOL parameter.

When VOL=REF is used, the &ALLVOL and &ANYVOL ACS read-only variables are set to 'REF=SD,' 'REF=ST' or 'REF=NS' as appropriate. Additionally, if the reference is to a data set on an SMS-managed volume, the storage group of the referenced data set is provided in the &STORGRP read-write variable, if it is available. (For some references to data sets on SMS-managed tape, it might not be, in which case the ACS routine should use the value of the &LIBNAME read-only variable instead.) If the referenced data set is new, it can still have multiple candidate storage groups because it has not been allocated yet. In that case only the first candidate storage group is passed in.