A roll-up summary field is a type of field that calculates values from related records and displays them on the parent record. A roll-up summary field can be used to count, sum, min, or max values from child records. You can create a roll-up summary field on an object that has a master-detail relationship with another object. In this case, you can create a roll-up summary field on Room that counts the number of Equipment records related to it. References: https://help.salesforce.com/s/articleView?id=sf.fields_about_roll_up_summary_fields.htm &type=5 https://help.salesforce.com/s/articleView?id=sf.fields_defining_field_types.htm &type=5