A file used within a PL/I program has a PL/I file name. A data set also has a name by which it is known to the operating system.
PL/I needs a way to recognize the data set(s) to which the PL/I files in your program refer, so you must provide an identification of the data set to be used, or allow PL/I to use a default identification.
You can identify the data set explicitly using either an environment variable or the TITLE option of the OPEN statement.