All variables in an Open PL/I program (except those under the control of the DEFAULT statement) must be declared. Each name, except the name of a built-in function, must be declared using a DECLARE statement (or if it is a label, by using the label as a statement prefix).
Each declaration is associated with a scope or region of the program in which a reference to the associated name is valid. The scope of a declaration includes the block in which it is declared and all nested blocks, except those in which the name has been redeclared.
Identifier attributes used in DECLARE statements are specified by either keyword or syntax. A name declared with the EXTERNAL attribute has the same scope rule as any other name, except that the object identified by that name is unique throughout the entire program. All declarations of a given name that have the EXTERNAL attribute identify the same object. Only files, static variables, and names of external procedures can have the EXTERNAL attribute. Files and procedures acquire the attribute by default, but static variables have internal scope unless they are explicitly declared with the EXTERNAL attribute.
A given name cannot be declared more than once within the same block, unless it is declared as the name of a structure member. In that case, the name may be redeclared within the same block, provided no two members at the same level of the same structure have the same name. See the help topic Scope for more information.