Used for: adding extra text/numbers to the end of an existing field.
With the "append value" rule you can add a certain text or number of your choice to the end of an existing field. For example, you could use it to add promotional text to the end of every title. How this works is shown in the example below.
The rule in the screenshot above will append the text "Now on sale!" to every product title, which results in the following output.