Why do messages to and from some contacts are noted with '«««' and '»»»', while for some others '<<<' and '>>>' are used?

    (0) The style of those indicates the transport mechanism used. '<<<' and '>>>' are non-acknowledged messages through the server, '«<<' and '>>»' are acknowledged messages through the server, '«««' and '»»»' are acknowledged messages via a direct connection, and finally '»%»' and '«%«' acknowledged messages via an SSL-encrypted direct connection.