A system lockup is defined as being one in which all stations are locked and not useable; it should be distinguished from a station lockup, in which one or more of the stations (but not all!) lockup.
From bad to worse:
Lockup only lasts 5 seconds.
Lockups lasts 2 minutes or longer.
Only one virtual on a terminal is dead.
One terminal (all virtuals) is dead.
All terminals are dead.
All terminals & main console dead, but clock running.
All terminals & main console dead, but clock stopped, reboot ok.
All terminals & main console dead, needs power off or reset.
When does it happen?
After running all night, dead the next morning.
After starting or during a print job to the main console parallel printers.
During a print job to a terminal's printer.
Just sitting, not doing a thing.
During a window switch.
After changing some piece of hardware or software.