This directory /usr/share/ca-certificates/trust-source/ contains CA certificates
and trust settings in the PEM file format. The trust settings found here will be
interpreted with a low priority - lower than the ones found in 
/etc/ca-certificates/trust-source/ .

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QUICK HELP: To add a certificate in the simple PEM or DER file formats to the
            list of CAs trusted on the system:

            Copy it to the
                    /usr/share/ca-certificates/trust-source/anchors/
            subdirectory, and run the
                    update-ca-trust
            command.

            If your certificate is in the extended BEGIN TRUSTED file format,
            then place it into the main trust-source/ directory instead.
=============================================================================

Please refer to the update-ca-trust(8) manual page for additional information.