这个枚举根据其物理表现定义了不同类型的空间边界。空间边界可以是物理分割，也可以是虚拟分割。
IFC2.0中增加的新枚举
| Constant | Description |
|---|---|
| PHYSICAL | The space boundary is provided physically (by a physical element). |
| VIRTUAL | The space boundary is provided virtually (by a logical divider that has no physical manifestation). |
| NOTDEFINED | No information available. |