A PL/I program can use a library directly. If you are adding a new member to a library, its directory entry will be made by the operating system when the associated file is closed, using the member name specified as part of the data set name.
If you are accessing a member of a library, its directory entry can be found by the operating system from the member name that you specify as part of the data set name.
More than one member of the same library can be processed by the same PL/I program, but only one such file can be open as output at any one time. You access different members by giving the member name in a DD statement.