Programmers utilize various access methods (such as QSAM or VSAM) in programs for reading and writing data sets.
Access method depends on the given data set organization.
PDS/E structure is similar to PDS and is used to store the same types of data.
However, PDS/E files have a better directory structure which does not require pre-allocation of directory blocks when the PDS/E is defined (and therefore does not run out of directory blocks if not enough were specified).
Each member can be accessed directly using the directory structure.
Typed datasets create an auto-generated class inherited from the dataset base class.
Whenever a member is deleted, the space it occupied is unusable for storing other data.
Likewise, if a member is re-written, it is stored in a new spot at the back of the PDS and leaves wasted “dead” space in the middle.
They are most often used for storing multiple JCL files, utility control statements and executable modules.
An improvement of this scheme is a Partitioned Data Set Extended (PDSE or PDS/E, sometimes just libraries) introduced with MVS/XA system.