Different disciplines express different meanings. For example, in control, wavy lines are added to indicate the estimated value of a value. For example, when designing an observer, if it is difficult for us to directly observe a certain state, we will design an observer to estimate the state at a certain moment through input and output values.
Actually, it's all rules These rules are not used after high school. Generally, it will be used in related courses of mathematics major. Generally, it is a concept derived from a set or concept, and wavy lines are added for difference. Mathematically, it is a mathematical symbol representing equivalence relation. In the final use (especially in lexicography), it is sometimes used as a font size.