Which prefix is used to reference an expression rule?

The prefix used to reference an expression rule is "rule!". This prefix is specifically designed to indicate that the referenced item is an expression rule, which can contain complex logic that calculates and returns a value. This is part of the syntax in Appian's Expression Language, where different prefixes are used to differentiate between types of components. For instance, "cons!" is used to reference constant values, while "expr!" and "ri!" are not standard prefixes associated with expression rules. Using "rule!" ensures that Appian interprets the reference correctly as an expression rule, allowing developers to leverage the logic defined within.