PL/I input and output statements (such as READ, WRITE, GET, PUT) let you transmit data between the main storage and auxiliary storage of a computer.

A collection of data external to a program is called a data set. Transmission of data from a data set to a program is called input. Transmission of data from a program to a data set is called output. (If you are using a terminal, "data set" can also mean your terminal.)

PL/I input and output statements are concerned with the logical organization of a data set and not with its physical characteristics. A program can be designed without specific knowledge of the input/output devices that are used when the program is executed. To allow a source program to deal primarily with the logical aspects of data rather than with its physical organization in a data set, PL/I employs models of data sets, called files. A file can be associated with different data sets at different times during the execution of a program.