It is changing a file (creating a UCS) that isn't the primary file, just my copy of it. In my view this a feature that is conceptually broken. It wants to create a UCS in that file and save it so that an AutoCAD user can reference it, making it possible to line up this file with the survey data if those files are combined in AutoCAD. Revit however regards this positioning choice as significant and when we try to save our work it prompts us to save the changes to the linked DWG file too. Revit generates a warning (refer to the other post's images) that this project isn't sharing coordinates with this file so it will import it using the WCS of the file, which is still pretty arbitrary in this scenario. If they just started designing without a background file from us (not really likely, what context would they have?) then we could link their file using Positioning: Auto - by Shared Coordinates. If true then we'd be better off importing their file using Positioning: Auto - Origin to Origin. In my opinion it is much more likely that they started designing based on a plan we exported from Revit and sent to them.
That means their file and the survey file don't share in their understanding of how to start drawing relative to the WCS in their AutoCAD files. It isn't very likely that they started designing by importing the survey DWG. Now imagine we link a DWG file that, to pick a discipline, is the fire protection design. It moves the Survey Point (when the icon is clipped) to mark the WCS (World Coordinate System) origin location of the linked file. Revit establishes a relationship with this external file and moves our project's shared coordinate system to align with the survey file.
Imagine we link a survey file into our Revit project and use Acquire Coordinates. However that seems like a rarer circumstance to me, especially if Revit happens to be the primary platform in use. I can imagine how it might be useful if we are mixing a variety of trade files that might not have discussed how their work relates to the WCS origin. In my view, for multiple survey file relationships, that's most likely not necessary and just creates confusion.
In other words we agree as users to draw things relative to 0,0,0 in a specific way so our files will line up when we use them as an external reference. Revit can't know if these files have an agreed upon relationship to the WCS origin because in AutoCAD that's a subjective user based agreement, not something captured in code. If I understand their logic, it means that using Positioning: Auto-By Shared Coordinates when we link a file triggers a response in Revit that it needs to create a UCS (User Coordinate System) to permit AutoCAD users to align these files if they are combined (apart from Revit) in AutoCAD at some point.
This means they (Revit's developers) are assuming the files we are linking don't already share an agreed upon file alignment strategy between the people that created them. This means what I wrote about in the other post is happening on purpose, not a bug. As the title suggests, regarding an earlier post earlier post, recently I was participating in a thread at the Autodesk User Groups and an Autodesk support person wrote that Revit intentionally attempts to create a User Coordinate System.