This directory /usr/share/pki/ca-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/pki/ca-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/pki/ca-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 source/ directory instead.

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