How does the Rule Engine node work?

It applies conditional logic, the equivalent of CASE in SQL. Column names go in $ signs, and each rule uses => to give a result.

$Revenue$ > 1000 => "High"
$Revenue$ > 100  => "Medium"
TRUE             => "Low"

Rules evaluate top to bottom and the first match wins, so order from most specific to least — putting > 100 first means > 1000 never fires. End with a TRUE catch-all, or unmatched rows get a missing value.

For arithmetic use Math Formula, and for text use String Manipulation; Rule Engine is for conditional assignment.