Its not occupancy status, its "Type". You can check if two attachments are compatible that way, by comparing their type strings.
The true big difference between attachments and docking ports is in their behavior in Orbiter: Attached child vehicles have no effective mass, no thrust. Docked vehicles have. If you dock a vessel to an tug stage for example, the vessel will offset CoG, its thrusters can still move the tug and the tug will have to push the mass of the vessel. If you attach it, the tug will only look different, but the flight dynamics of the tug did not change that way.
The true big difference between attachments and docking ports is in their behavior in Orbiter: Attached child vehicles have no effective mass, no thrust. Docked vehicles have. If you dock a vessel to an tug stage for example, the vessel will offset CoG, its thrusters can still move the tug and the tug will have to push the mass of the vessel. If you attach it, the tug will only look different, but the flight dynamics of the tug did not change that way.