“Every year the king of the country of Ta-shih (the Arabs of the Khalif empire) who is styled Su-tan (Sultan) sends tribute-bearers, and if in the country some trouble is apprehended, he gets the Ta-shih to use their military force in restoring order.... The products of the country consist in opaque glass, corals, raw gold, brocades, sarcenets, red cornelian stones and real pearls”

- Chinese official letter documenting the Song and Yuan dynasties, translated and published in China and the Roman Orient: Researches Into Their Ancient and Mediaeval Relations as Represented in Old Chinese Records, published 1885

This passage BEST demonstrates which of these?