How can we restrict the notifications for user specific

In the first glace, the plugin is looking very awesome.

But after installation, I have created a notification. but this notification is showing to all the users. Although there is a field “Show to users” but don’t know how to use it. Can you please give me the instructions on what data we need to use in this field. i.e., user id or username.

Also, what if we have to a notification to all the subscribers. Is there any option to show the notification for all the user based on their role as well?

Thank you in advance.