When executing commands in the background and foreground concurrently, you should be aware that allocation of a data set both in the foreground and the background might not be successful. If a data set has been allocated (foreground or background mode) with a disposition of OLD, MOD, or NEW, it cannot be allocated in the opposite mode by the ALLOCATE command or any other command processor. For example, the ALLOCATE command, issued in the background for a data set in use in the foreground, will issue an error message that the data set is already in use. Any command remaining in the job stream is processed.

If a user's LOGON procedure allocates a data set with a disposition of NEW, MOD, or OLD, a background job specifying the same LOGON procedure will not execute until the user logs off or frees the data set in the foreground. Similarly, if a background job is executing and the user attempts to log on a terminal with the same LOGON procedure, the logon attempt fails.