When linking models together, you can make the host project recognize the Room Bounding parameter of elements in the linked model.
This setting allows you to place rooms whose boundaries are defined by elements in both models.
The status bar displays the following:
RVT Links: Linked Revit Model: <model name>.
Now you can place rooms in the plan view, using the boundaries defined by room-bounding elements in the host project and the room-bounding elements in the linked model.