Impact refers to the number of people affected, or likely to be affected by, a particular Incident. Urgency refers to how quickly the Incident is likely to cause serious interruptions to services (and thus how quickly it must be fixed).
For example, if a large network is undergoing an upgrade and is thus slower than normal, then there will be a high impact (i.e. everyone on the network is affected) but a low priority (nobody is seriously affected by the issue). If the hard drive on one PC has a fatal crash, then that would have a low impact (one person), but a high priority (it will quickly and seriously impair the service).
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