When you declare or open a file in your program, you are describing to PL/I the characteristics of the file. You can also use a DD:ddname environment variable or an expression in the TITLE option of the OPEN statement to describe to PL/I the characteristics of the data in data sets or in PL/I files associated with them. See Associating a PL/I file with a data set for more information.
You do not always need to describe your data both within the program and outside it; often one description serves for both data sets and their associated PL/I files. There are, in fact, advantages to describing your data's characteristics in only one place. These are described later in this chapter and in following chapters.
To effectively describe your program data and the data sets you are using, you need to understand something about how PL/I moves and stores data.