Has it occurred to anyone to just look at the JCL Reference manual?
" When you specify DISP=MOD or DISP=NEW for a partitioned data set (PDS) or
partitioned data set extended (PDSE), and you also specify a member name in
the DSNAME parameter, the member name must not already exist. If the member
name already exists, the system terminates the job.
When you specify DISP=OLD for a PDS or a PDSE, and you also specify a member
name in the DSNAME parameter, the data set must already exist. If the member
name already exists and the data set is opened for output, the system
replaces the existing member with the new member. If the member name does
not already exist and the data set is opened for output, the system adds the
member to the data set.
When you specify DISP=MOD for a PDS or a PDSE, and you do not specify a
member name, the system positions the read/write mechanism at the end of the
data set. The system does not make an automatic entry into the directory.
When you specify DISP=MOD for a PDS or a PDSE, and you do specify a member
name, the system positions the read/write mechanism at the end of the data
set. If the member name already exists, the system terminates the job.
When you specify DISP=SHR for a partitioned data set extended (PDSE) and
also specify a member name, then:
* If the member name exists, the member can have one writer or be shared
by multiple readers, or
* If the member name does not exist, the member can be added to the data
set. Thus, multiple jobs can access different members of the data set and
add new members to the data set concurrently -- but concurrent update access
to a specific member (or update and read by other jobs) is not valid.
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