This product-specific custom field is displayed above the price in the buy box on the product page, e.g. "Special price".
Note: The custom field is displayed when "Show price label" in the theme configuration is set to "Custom field" or "Custom field or snippet". With a list price, the setting "individual snippet" replaces it.
Custom field
You can find the "Price label" custom field in the respective product:
Tab "Specifications" ⇒ Block "Custom fields" ⇒ Tab (Set) "Products (ThemeWare® ... Pro)"
Type: Text
Technical name: twt_{theme}_{edition}_custom_field__product__price_label
Further information
You can find more information about the "Price label" extension here:
Price label