A business rule runs server-side when its configured database operation and conditions match. A before rule can validate or set values before a record is saved. An after rule works after the database operation when related processing needs the saved record. Asynchronous work runs later and shouldn't be treated as an immediate form response.
Choose the timing from the requirement. Updating the current record inside the wrong rule pattern can cause repeated execution or unnecessary writes. Keep conditions narrow and avoid expensive queries for each row when a set-based or simpler approach fits. A client script can help a user fill out a form, but server rules are needed for requirements that must apply to imports and integrations too. This lesson checks execution choices and common failure modes.