Pushnotification message is nothing but marketer will send the message or notification to the mobile app through pega marketing.
User is some one who is using that mobile app. PegaMarketung uses external third party extensions to send the notifications to mobile app.
To implement this,first we need to configure proper google key under AppStudio->settings->channels.
Then the mobile app must be installed and that particular mobile device must be registered to receive the notifications. You can check the registered devices by checking the instances of class 'Data-PushNotification-Delivery-Registration'.
Now for sample test case, you can use the activity 'PushNotificationForToken' to send notifications.