Data Policies are interesting in that they truly stop things from happening.
What is a Data Policy?
Data policies let you configure conditions on a table and related fields that are required or read-only. When anything tries to change those fields, the data policy is checked and the change is either allowed or denied regardless how you do it.
When to use a Data Policy?
Data policies are great for enforcing specific details and there's even an option to run it as a UI Policy in addition to the server side check.