A hospital has a patient form that they share with their patients over Gmail.

The appropriate rule type to use in the DLP profile for a document that is considered confidential when filled out is fingerprint classification. Fingerprinting is a method used to identify and protect sensitive data within documents. It works by creating a digital fingerprint of a file, which can then be used to detect any copies or derivatives of that file. In this case, fingerprinting would allow the hospital to differentiate between the blank patient form, which can be freely shared, and the same form with patient information filled out, which is confidential1.