In general, your Push Notifications will go to all your app users.
You can also create categories for your users to opt-in to, so that you can send targeted Push Notifications to relevant groups.
This is all created and controlled via your Dashboard.
Categories are great for creating specific groups for messages relating directly to them. Maybe its a particular group for your shop that wish to receive promotions only relating to 'menswear'; maybe you're a sporting club with each team wishing to receive alerts specific for them; or maybe its a group of users that wish to receive daily promotions.
You can create groups within your Dashboard, and users can opt-in via their app.
If no group is created (or set at the time of sending the Notification), the Push Notification will be sent to all app users that have given your app permission on their mobile device.
Your app users must select your app as a 'favourite' in the Directory in order to receive Push Notifications.