Generating Self-Signed Certificates
A self-signed certificate is useful for testing purposes or as a temporary solution until a CA-signed certificate is available.
If you are connecting to the PLC for the first time and no valid certificate for communication with this PLC is installed on your computer yet, then you will be prompted to create a self-signed certificate. You can also have this entered in the local Windows Certificate Store
when you log in.
See the instructions: Establishing a connection to the controller, logging in, installing a trusted certificate for encrypted communication