What is Useful Spirit?

The line in a hexagram that represents the subject of the querent's question. Selecting the correct useful spirit is the first step in Liu Yao analysis. For wealth questions, it is Wife's Wealth (妻財). For career or exams, Officer/Ghost (官鬼). For health, Offspring (子孫). For documents or housing, Parents (父母). The useful spirit's strength determines the overall verdict.

Source: Bushi Zhengzong (卜筮正宗), Wang Hongxu, 1709