In order to compile object-oriented COBOL programs, you will need an external library (which is logically one library) called a “repository”, which contains the required definitions of the program interfaces, classes and other interfaces. Even for programs that are not object-oriented, a repository is needed whenever interfaces are to be checked in a CALL (see Format 3 of the CALL statement in the “COBOL2000 Reference Manual” [1]). These definitions are read by the COBOL compiler so that additional checks can be run on the compilation unit itself with the goal of preventing runtime errors.
A repository need not be a single physical library; it could also consist of many libraries in a hierarchy, as in the case of COPY libraries.
Repository data includes both input and output data.