The Assignment Rule checkbox is a checkbox that appears on manual case creation pages when assignment rules are defined for cases. The Assignment Rule checkbox determines whether or not to apply assignment rules to manually created cases. If the Assignment Rule checkbox is selected by default, then any manually created cases will be assigned according to assignment rules instead of being owned by the agent creating them. To prevent this from happening, an administrator can either deselect the Assignment Rule checkbox when creating cases manually; or change the default setting for this checkbox under setup by selecting or deselecting Use active assignment rules by default. Reference: https://help.salesforce.com/s/articleView?id=sf.customize_casesupport_assign.htm&type=5