A roll-up summary field is a type of field that calculates values from related records, such as the count of child records or the sum of a field on child records. In this case, the administrator can add a roll-up summary field to the open house object to track the number of visitors by counting the child records on the visitors object.References:https://help.salesforce.com/s/articleView?id=sf.fields_about_roll_up_summary_fields.
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