Several elements of COBOL require special handling when XFDs are built. These include multiple record definitions REDEFINES, FILLER, and OCCUR. This section describes how the compiler handles each of these situations.
Note that in many instances you can override the default behavior described below by placing special comment lines in the FDs of your COBOL code. These comments are called XFD directives, and are described in Using XFD Directives. For example, the WHEN XFD directive lets you use multiple definitions for a single set of data by specifying when each definition should be used.
Databases generally do not support the notion of multiple definitions for the same column. As the following paragraphs explain, whenever a COBOL program gives more than one definition for the same data, the compiler chooses which definition to use in the XFD and disregards the rest.