It is up to you to decide if an object should run in exclusive, foreground or background mode. If an object manipulates user interface, it should be definitely set to exclusive or foreground mode. On the other hand, if an object executes all its actions in background and several instances can run in parallel, it might be set to run in background mode. This then allows you to run some processes in parallel on the same PC - i.e. you drag two processes on to the same PC in the control room and let them run at the same time.
Also note that if a process uses an object that is set to run in exclusive mode, no other process will be able to run on the same PC at the same time. If you want to run several processes at the same time on the same PC, only one of the processes can use objects that run in the foreground mode, the remaining processes can only use objects that run in the background.