Liber 777, Table V, CXXXVII (Exalted vs Ruling?)
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These are two different dignities that planets have in signs. They mean different things in different contexts.
In astrology, there is functionally almost no distinction: Both exalted and ruling planets are those planets whose natures most resemble those of the planet. Thus, Libra is ruled by Venus and has Saturn exalted: This means that the nature of Libra resembles the combined nature of Venus and Saturn. (The actual distinction between ruler and exalted has to do only with the origins of the two separate lists.)
In Qabalah, alchemy, and other fields that adapt astrological symbolism to their own non-astrological purposes, a distinction is often made that the sign of a planet's exaltation is it's highest or best expression.