Used for: replacing a value in a certain field with another value. It can also be used for deleting a value (replace by nothing).
With the "replace value" rule you can look for a certain value in a particular field and replace it with another value of your choice. If you don't enter a replacement value, you replace the value you search for with nothing (=deleting the value). Also, if the value you want to replace is not found in the field you select, the rule will do nothing, so it will not randomly add the replacement value in that case.
You could, for example, use the "replace value" rule to change your Dutch product URLs to German product URLs. This example is shown in the screenshot below.
Besides searching for a value and then replacing it with another value, the "replace value" rule can also be used to delete certain values from a field. In this case you just leave the "replace with" field (".de" in the example above) empty. The screenshot below shows an example in which the brand is deleted from the title of every product.