A truncated transliteration. The full Spanish region 'Castile' is rendered
卡斯蒂利亚 (Kǎsīdìlìyà), so
卡斯蒂 is often the first three syllables of a longer name (Castiglione, Castillo, Castilho).
卡 ('kǎ') and
斯 ('sī') are workhorse transliteration characters;
蒂 ('dì', 'stalk of a flower') typically renders the syllable '-ti-' in foreign names.