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HU - head unit, the dash mounted radio/cd/cassette player.
RTI - road traffic information. Provides mapping and navigation. Consists of base unit in the trunk of the car and motorised screen in dash.
MELBUS - (Mitsubishi Electric BUS)?
Communication interface found in Volvo cars and used for communication between HU and various audio sources- CD changer, RTI unit, TV tuner and others.