List of additional portals that should be added to the environment.
The directory where the portal definitions have been merged together (likely ~/.nix-profile/share/xdg-desktop-portal/portals) will get passed to xdg-desktop-portal.service via the NIX_XDG_DESKTOP_PORTAL_DIR environment variable.
Portals allow interaction with system, like choosing files or taking screenshots. At minimum, a desktop portal implementation should be listed.