Used for: replacing the complete value of a field, for another value of your choice. It will only replace the value if it's an exact match.
Used in: "THEN"-statements and "ELSE"-statements.
This rule will do an exact match on the input value and only replace it when there is an exact match of the complete field value. For example if the lookup value is "Red" and replacement value is "Brick" then the output will be "Brick" only for the products that have the exact input value "Red". It will not change products that have values like this "Dark Red", even if they contain "Red".
If you select the Case sensitive option, it will replace the fields that contain exact values of upper and lower case letters. So if this is active (blue check mark) and we look for the input "red", and our replacement value is "Brick", the output will be "Brick" for all products that have the value "red", but not for those that have the value "Red".