Occupancy means that a special feature can only be controlled by a unit that is within it. A unit can occupy a special feature by moving into base contact with it during the Remaining Moves sub-phase of its Movement phase. Once its movement is complete, the unit is placed within the special feature. Whilst a unit occupies a special feature it:
Cannot charge.
Can shoot (and cast spells) and is considered to have a 360° vision arc from the special feature itself.
Can be shot at (and be targeted by spells) by a unit (or Wizard) that can draw a line of sight to the special feature. However, the unit is considered to be behind full cover.
All measurements to or from a unit that occupies a special feature are made to or from the base of the feature, rather than the individual model(s).
Special features do not tend to be particularly large. Therefore, they can only be occupied by a single unit.
Unless it is fleeing, a unit occupying a special feature can leave it during the Remaining Moves sub-phase of its Movement phase, by making a follow up move, or by pursuing an enemy. Simply place the unit in formation, so that at least one model is in base contact with the feature. The unit's movement then ends.