Flatpak's sandboxing features limit the access applications can have to host resources, such as the filesystem, system services, network, bluetooth, and more. These static permissions are set by application maintainers and can be overridden by users. However, to limit the permissions granted to apps, they need to be able to dynamically request resources in response to user actions. For example, the document portal allows apps to only be granted access to files chosen by the user in an interactive dialog. There are also portals to allow apps dynamic access to printing, screenshots, screencasting, remote desktop, camera, and more. The interactive nature of the portals means that the user is always in control of what access is granted.

The xdg-desktop-portal project is

There is also a library intended for applications to make the API easy to use; see the documentation here: https://libportal.org/

This project can fund development work on portals, such as writing new portals and maintaining the existing ones.