The policy severity determines the frequency with which alerts are generated.
The severity levels are:
Severe – An alert may be generated once an hour.
Warning – An alert may be generated once every 24 hours.
Informative – Only one alert is generated; no additional alerts are generated, even if the policy criteria continue to be met.
See Alert Policy Examples for examples detailing the effects of the assigned severity levels.
Alert Severities for Exchange and Media Validation are Irrelevant
Because exchanges and media validations are events, not resources, they generate alerts whenever a new exchange or validation is processed and the policy criteria are matched, regardless of time frames.
Therefore, the severity levels you assign to these alert policies are irrelevant. See Policy for Exchanges Using "Media Health Indicator" Example and "Warning" Policy for Media Using "Media Health Indicator" Example below for details.
Additionally with exchange and media validation alert policies, you must take care not to create overlapping policies that might generate multiple alerts from the same exchange or validation. See Avoid too many alerts for details.