Hi @Ronald-Bolijn and @Bob-Mooij ,
The options presented by @Bob-Mooij are correct in the documentation however a third option is available.
Where we describe multiple applications in the documentation, you can register an application per authorization / user account of a project. You have access to two of such users per project; one is invisible and used by the portal and the second is the user that is provided by with the API credentials in your project.
Per user you can manage the serialnumbers of the subscriptions as described here.
This option is more applicable when you want to sent data from the same devices to different applications.
Where one application could be subscribing to the lifecycle events and another application subscribes to the payload resources
So concluding advice
The preferred way for grouping devices to applications is trough creation of separate projects.
But if you need data from the same device distributed to more applications this can be done using this approach.