Stephen was the first to begin striking monies with his own initials on them and not those of the Byzantine Emperor. He was not attached to the empire in any way: neither by appointment nor by any other ties except heritage of his dukedom.
Stephen was the pioneer in marking coins with his personal mark instead of those of the Byzantine Emperor.  He held no formal position within the empire, nor did he have any other ties to it beyond being a member of the dukedom.