Revit: Cannot Tag Room in a Linked Revit Model

I ran into a case of this today, and as I checked a number of other things that were not the cause of the problem before I found this Autodesk^® Knowledge Network article, I thought I would document it here so I can find it faster in the future.

The problem was that the linked model did not have any Rooms in the equivalent phase that was set in the host file Views in which the Room Tags were to be placed. Since Rooms are phase-specific, they need to be recreated for each phase.

In this particular case, there were variations in the names that were given to the phases. To verify how the phases for a particular linked file are mapped (and make any necessary adjustments):

1. In the host file, select the linked model.
2. On the Properties palette, select the Edit Type button.
   
3. In the Type Properties dialog, under the Other category, select the Edit button to the right of the Phase Mapping parameter.
   
4. In the Phases dialog, you can assign the appropriate phase from the linked file to each phase in the host file. Revit will usually match up phases with the same name. (I am not sure what happens if the same names are used, but the phases are in a different order. That is such bad practice that I do not even want to test it.) Selecting in the Phase from linked file column next to a given host Phase will activate a drop-down list, which can be used to select from the phases defined in the linked file.
   

The phases shown in the image above are just the default Existing and New Construction phases, and are the same in both sample files. On a real project, particularly one with multiple phases, there are "opportunities" for different disciplines to use different names and it may not be readily apparent how the phases map without checking.