System executable files are in an operating system format. They can be shared between processes. If multiple copies of the same system executable are run at the same time, the operating system shares the procedural code and creates separate data areas for each process. This includes sharing any COBOL procedural code linked into the system executable.
System executable files are loaded by the operating system, which creates a new process in which to run it. They cannot be called from COBOL using the CALL statement.
System executable files do not typically have a filename extension, and are sometimes known as a.out files.
Programs are compiled using the cob -x command; see cob (cob32 or cob64) Command and Linking to System Executables for details.