This carries the specific reason a CA was determined to be invalid, as reported in a security notification. Having the actual reason text, rather than just a generic CA-invalid flag, lets an admin immediately tell whether the fix is re-importing an expired root CA, correcting a chain-of-trust issue, or something else entirely, without needing to dig through device logs separately to find out why. For example, a notification carrying this as CA certificate expired points directly at renewing the root or intermediate CA certificate, versus a reason like signature verification failed which would instead point at a corrupted or tampered certificate file.