Exam Certified Platform App Builder topic 1 question 272 discussion

ANSWER: BCD https://help.salesforce.com/s/articleView?id=sf.fields_about_roll_up_summary_fields.htm&type=5 B. Because Roll-up Summary Fields are not displayed on edit pages, you can use them in validation rules. C. Once created, you cannot change the detail object selected or delete any field referenced in your Roll-up Summary definition. D. Validation errors can display when saving either the detail or master record. NOT: A. Automatically derived fields, such as current date or current user, are allowed in a Roll-up Summary Field. FALSE: "Automatically derived fields, such as current date or current user, AREN'T allowed in a roll-up summary field." E. Advanced currency management has no affect on Roll-up Summary Fields. FALSE: "Advanced currency management AFFECTS roll-up summary fields."

So, validation errors can display when saving either the detail or master record. ... When you add a child record, your validation rule on the child object can check if the count is already 25 or greater.

Because roll-up summary fields are not displayed on edit pages, you can use them in validation rules but not as the error location for your validation. Avoid referencing a roll-up summary field from a child record.