Most avionics unit have both a part number and model number. Some models will have several part numbers representing technically and functionally different units available. An example of this might be the King KNR-634 Navigation Receiver which has several versions. Some part numbers of the KNR-634 include marker beacon receiver and glideslope receiver boards while some may not have these boards at all. Some other instances of this may be panel mount units such as the KX-155 which may or may not contain such features as glideslope receiver and isolation amplifiers for audio. Therefore, a unit containing all available features for a certain model will generally have a higher cost than a unit that does not have the same features and function.