The new ILD
An internal wall air vent, or ILD, consists of an axial fan with sound insulation and two internal panels, each on the room side, with two separate filter holders.
The ILD is used if you have an interior room that must be ventilated via another room. So if there is no outside wall available in a living room, then one or more ILD can be used to create a coupling with other rooms and thus establish an active air connection.
For example, an e² ventilation system can be installed in a bedroom (=primary room) and an adjacent adjoining room (=secondary room) can be ventilated and deventilated via ILD.
The ILD is perfect for cascaded ventilation.
You can find more information about the ILD here.