What does the space in the middle mean?

It means hollow, that is to say, this place is empty within a certain range, without any buildings and structures. For example, a hotel enters the lobby, and the top of the lobby is very high, three stories high. Then on the drawing of this hotel, on the second floor plan, the location of the lobby is actually empty, so this place should be marked as hollow. In addition, garbage passages, ventilation pipes and other pipes are sometimes marked as hollow on the plan.