Function of the seat belt warning for the driver and front passenger
The 
seat belt warning lamp in the instrument display is a reminder that all vehicle occupants must wear their seat belts correctly.
In addition, a warning tone may sound.
The belt warning goes out as soon as the driver and front passenger fasten their seat belts.
If you are using an ISOFIX or i‑Size child restraint system on the front passenger seat, where the child is not secured using the seat belt of the front passenger seat, the belt warning may be activated. In this case, insert the seat belt buckle tongue into the belt buckle of the front passenger seat. Guide the seat belt between the ISOFIX or i‑Size child restraint system and the backrest of the front passenger seat.