The reason why Fulfillment Order does not appear as an option when creating a rollup field on the Order Summary object is that the Order Summary field on the Fulfillment Order object is a Lookup. A Lookup field is a type of custom field that creates a relationship between two objects, but does not support rollup summary fields. To create a rollup summary field, the relationship between the objects must be master-detail, which means that the child record cannot exist without its parent record. Verified Reference: https://help.salesforce.com/s/articleView?id=sf.custom_field_types.htm&type=5https://help.salesforce.com/s/articleView?id=sf.fields_about_roll_up_summary_fields.htm&type=5