Explanation:

To create a trainable classifier that can be used in an auto-apply retention label policy, you need to follow these key steps:
1. Create the trainable classifier
This is the first step where you define the classifier, specifying the types of content it should identify.
2. Test the trainable classifier
Before using the classifier in production, you need to validate its accuracy by testing it against sample documents to ensure it correctly classifies the intended data.
3. Publish the trainable classifier
Once testing is successful, you must publish the classifier so that it can be used in policies like auto-apply retention labels in Microsoft Purview.