Explanation
Your organization might use Windows Server FCI to identify documents with personally identifiable information (PII) such as social security numbers, and then classify the document by setting the Personally Identifiable Information property to High, Moderate, Low, Public, or Not PII based on the type and number of occurrences of PII found in the document. In Office 365, you can create a DLP policy that identifies documents that have that property set to specific values, such as High and Medium, and then takes an action such as blocking access to those files.
Before you can use a Windows Server FCI property or other property in a DLP policy, you need to create a managed property in the SharePoint admin center.
Reference:
https://docs.microsoft.com/en-us/microsoft-365/compliance/protect-documents-that-have-fci-or-other-properties