Custom Error element versus validation rules
Key Takeaway
A Custom Error element in a record-triggered flow can block a record change, including a delete, and show your own message. Custom validation rules still run.
The Custom Error element gives admins a flow-based way to enforce business rules. A record-triggered flow uses it to stop a record change and show the user a message you write.
What happens when a change is blocked
When a Custom Error element blocks a record change, the flow rolls back the change that triggered it. The change stays rolled back until the error is fixed. The message tells the user exactly what caused the error, and the user can then fix the problem and try again.