The Unit Number is the companion to the Address Number and it exists because most people in cities do not occupy a whole building. Where an address has an apartment, unit, flat or suite designation, this position takes that inner number and reduces it in the usual way. Letters within it are converted through the same table the name positions use, so 4B gives four plus two.
Practitioners generally read the two together and give the unit the greater weight, on the reasoning that the reading is meant to describe the space actually lived in. The building's number is then treated as the wider setting: the character of the address as a whole, within which the individual dwelling has its own. A household in unit seven of a building numbered four is described as a private and reflective home inside a stable and work-oriented block, and the reading is built from the relation between the two.
Where there is no unit designation the position simply does not apply, and the chart is left without it rather than filled from somewhere else. That is worth noting, because most positions in this system can always be computed from a name or a date. Both of the place positions can be genuinely absent, and a practitioner who fills an absent one by substituting a floor number or a door number is making something up rather than following the method.