The THREAD keyword indicates that the ILE RPG module being created is intended to run safely in a multithreaded environment. One of the major thread-safety issues is the handling of static storage. When multiple threads access the same storage location at the same time, unpredictable results can occur.
Specifying the THREAD keyword helps you make your module thread-safe with regards to the static storage in the module. You can choose between having separate static storage for each thread, or limiting access to the module to only one thread at a time. You can mix the two types of modules in the same program, or service program. However, you should not omit the THREAD keyword in any module that may run in a multithreaded environment.
You do not have to be concerned about automatic variables. Automatic variables are naturally thread-safe because they are created for each invocation of a procedure. Automatic storage for a procedure is allocated in storage which is unique for each thread.