When creating a new uptime check (whether a browser check, or a regular uptime check), you have the option to set Alert Priority.
High priority alerts are for alerts that must wake you up.
When a high priority alert fires, OnlineOrNot sends notifications to every location you've configured, particularly SMS.
Low priority alerts are for alerts that are more of an FYI for the team.
When a low priority alert fires, OnlineOrNot notifies you via Email, Slack and Webhook only.