Revit knows who we are based on a simple piece of data, the Username entry here (Application Menu (Big R) > Options). That is usually defined by who logs on the computer. If we log on our computer, then open Revit AND change the Username...Revit preserves this alternate username until we change it again. If we never change this value then Revit just uses the computer's log on username identity. When we change it Revit captures and preserves it for this particular person's log on credentials on the computer.
Using Worksets (Worksharing) this username provides our unique identity to Revit, the Librarian (not to be confused with the TV show or movies). We can't just arbitrarily change our username while we are working in our Local File. If we do that we'll be greeted with an unpleasant message when we attempt to use Synchronize with Central.
We can become convinced that changing the username is permissible because Revit doesn't complain until we attempt to use Synchronize with Central. It's also easy to think it is reasonable because when the file contains changes that have not been synchronized yet we see this message when we attempt to change the username. When Revit complains and offers the reason that changing the name isn't going to work it implies that there might be an acceptable circumstance.
Fwiw, we CAN change our username freely if we are working in a central file (discouraged). However we won't find that very fulfilling if other users are also working on the project via their own local files. We'll be advised by Revit that it is necessary to use Save As to create a our own Local File first, when we use Synchronize with Central.
We CAN assume any username identity, we just have to change the Username parameter before creating a new Local File.
Damn Revit's Worksets and Worksharing features are fussy!
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