An antimension is the cloth the priests unfolds on the altar before the Great Entrance (when we process in the Church with the Chalice and Paten). The Metropolitan signs the antimension which shows that the parish is a canonical parish under a canonical Bishop. In a manner of speaking it is the authorizing document, the means by which a parish is affirmed as authentically Orthodox.
Below is a letter received from the Archdiocese with more details.
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