A: Classifiers
This categorization method is well suited to content that isn't easily identified by either the manual or automated pattern-matching methods. This method of categorization is more about using a classifier to identify an item based on what the item is, not by elements that are in the item (pattern matching). A classifier learns how to identify a type of content by looking at hundreds of examples of the content you're interested in identifying.
Where you can use classifiers
Classifiers are available to use as a condition for:
Office auto-labeling with sensitivity labels
Auto-apply retention label policy based on a condition
Communication compliance
Sensitivity labels can use classifiers as conditions, see Apply a sensitivity label to content automatically.
Data loss prevention
E: Organizations have sensitive information under their control such as financial data, proprietary data, credit card numbers, health records, or social security numbers. To help protect this sensitive data and reduce risk, they need a way to prevent their users from inappropriately sharing it with people who shouldn't have it. This practice is called data loss prevention (DLP).
Reference:
https://learn.microsoft.com/en-us/microsoft-365/compliance/classifier-learn-about
https://learn.microsoft.com/en-us/microsoft-365/compliance/dlp-learn-about-dlp