Every module is composed of multiple classes, each with its own function and format. Some classes represent program text, the instructions and data that are loaded into virtual storage during execution. Other classes, such as an external symbol dictionary (ESD) and a relocation dictionary (RLD), are required for binding and loading the program. Additional classes, such as IDR and ADATA, provide descriptive information about the program module or its individual sections and are of use primarily for maintenance and debugging.

See Object and program module structure for the logical structure of elements appearing as one or more classes in a module.

Each element in the class represents the contribution of a single section to that class. The sequence of elements within the class is the same as the sequence of the sections within the module, specified on either the ORDER control statement or the ORDERS API function.

Classes are identified by class name. Unlike section names, which are assigned by the source language programmer, class names are normally assigned by an IBM® compiler or binder. Class names are a maximum of 16 bytes in length. Binder-defined class names begin with “B_”. Compiler-defined class names begin with “C_”. User-defined class names should not use these prefixes and should be no more than 14 characters long. Class names are not normally required on binder control statements, but can appear in listings and diagnostics. Each separately named class has a specified or an implied set of binding and loading attributes.