Promotional Text and Subtitle are the new feature in iTuneconnection which is available in iOS 11.0 or above. Subtitle is optional. But Promotional Text is not mentioned as optional. Which text we need to write as Promotional Text. Is it for only current app or any other apps using same developer account?
Promotional Text (or Description)
it is optional we can update the description on any time.
Your app’s promotional text appears at the top of the description and is limited to 170 characters. You can update it at any time without having to submit a new version of your app. Consider using promotional text to share the latest news about your app, which can include new promotions for your app or in-app purchases, upcoming features or content, limited- time sales, or other events within your app.
for more information you can get appstore product page and here also