It's like "emergency surgery." Your doctor can say, "We're scheduling you for emergency surgery in two days or that aneurism could burst and kill you. In the me

It's like "emergency surgery." Your doctor can say, "We're scheduling you for emergency surgery in two days or that aneurism could burst and kill you. In the mean time don't so anything stressful." So the emergency maintenance is maintenance that, if put off, could result in the complete collapse of the system so better to do it right away and have an annoying but hopefully brief impact.