Assignment rules are tools that allow administrators to automatically route records to users or queues based on certain criteria. For example, anassignment rule can assign cases to different queues based on case priority, origin, type, or other fields. Assignment rules can be triggered when records are created manually, via email, web, or API. Assignment rules consist of multiple rule entries that define the criteria and actions for each assignment scenario. References: https://help.salesforce.com/s/articleView?id=sf.customize_leadrules.
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