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.htm&type=5

To use as an identifier whenimporting and updating records from a separate system, an administrator should use External ID field type on an object. External ID fields allow storing unique identifiers from external systems and using them for matching records during import or update operations. External ID fields can also be used for upsert operations that insert new records or update existing ones based on external ID values. For example, an administrator can create an External ID field on Account object that stores account numbers from an external ERP system and use it for importing or updating accounts from that system. Rich Text field, Record ID, and Auto-Number field are not suitable for using as identifiers when importing and updating records from a separate system. References: https://help.salesforce.com/s/articleView?id=sf.
fields_about_field_types.htm&type=5https://help.salesforce.com/s/articleView?id=sf.data_loader_upsert.
htm&type=5

Section: Volume H