Test Outside Lock for New Key
If this check box is selected, before creating the critical region lock (outside the section of code controlled by the lock), the system checks whether the new row already exists. If it does, nothing happens and the lock is never obtained. If the row does not exist, the lock is obtained, the NextKeys row is added normally, and then the lock is released.
If this check box is cleared, the system does not check whether the new row already exists until after obtaining the critical region lock. If the row does exist, nothing happens, but the lock is briefly held and then released. If the row does not exist, the NextKeys row is added normally and then the lock is released.