The MFJ-916B duplexer is a series of inductance coils and capacitors that acts as a band-pass filter. Your MFJ duplexer can be used two different ways. The duplexer combines the output of the HF/VHF transceiver and the output of the UHF transceiver so that both transceivers can be connected to the same antenna. The duplexer can also be used to separate HF/VHF transmissions from UHF transmissions so that one transceiver can be connected to two antennas.
The MFJ-916B is designed for indoor use. If you use the MFJ-916B outdoors, it should only be used for short periods of time under good weather conditions. Do not use the duplexer outdoors when there is any kind of precipitation.