The easiest way to see where the cross-module association (in this case Device_User) goes to, is to right-click the association and selecting 'Go to other side’. By the look of things, the association refers to the System.User entity.
Regarding saving extra information for the user, you can create a specialization of the System.User entity, which allows you to store more data regarding users in a seperate entity while still being able to use all the functionality of the System.User entity. In this entity you can add an extra attribute to keep track of the amount of received push notifications.
With kind regards,