JES Symbol Service (IAZSYMBL) symbols are defined at either the task level or the job step level. JES symbols that are defined at the task level are only visible to the code that is running in the same task (TCB). Symbols that are defined at the job step level are visible to the code that is running in all tasks in the same job step. A JES symbol at the task level overrides a JES symbol with the same name at the job step level.

The JES Symbol Service manages JES symbols by using the CREATE, CLEAR, UPDATE, DELETE operations. In addition, the EXTRACT operation provides access to JCL Symbols, unless the JSYMLVNJ option is specified. When access to JCL symbols is required, the caller must be in the task mode and cannot hold any locks. When access to a JCL symbol is requested, the JCL symbol is only returned if no JES symbol with the same name was found.

The CREATE operation requires the symbol definition level to be selected by setting either the JSYMLVLT or JSYMLVLJ option, but not both.

The CLEAR operation requires the symbol definition level to be selected by setting the JSYMLVLT or JSYMLVLJ option, or both.

The UPDATE, DELETE and EXTRACT operations ignore the symbol definition level selection. Processing for the these operations always starts at the task level and only moves to the job step level if the requested symbol was not found at the task level. If the EXTRACT function does not find a JES symbol with a particular name, it will search for a JCL symbol with the same name.

The JSYMLVUD option modifies the behavior of a CREATE operation request when duplicate symbols are encountered. If the JSYMLVUD option is specified, duplicate symbols are updated. If the JSYMLVUD option is not specified, duplicate symbols are not processed and a duplicate symbol warning is returned.

The JSYMLVJC option applies JCL constraints during EXTRACT operation processing. If the JSYMLVJC option is specified, only symbols that can be used for JCL substitution are returned. The JCL constraints are a maximum symbol name length of 8 characters, and a maximum symbol value length of 255 characters. Rules for JCL Symbol Service (IEFSJSYM) names and values are documented in z/OS MVS Programming: Authorized Assembler Services Reference EDT-IXG.