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When a common VI is used in one or many projects, it is controlled by SCC so you do not have to have add it explicitely to SCC. If the VI is not part of any SCC project, there is little need to control this VI...
However, what you can do is to create a VI with all the common VIs on its diagram and make a project with this VI as Top Level.
All VIs controlled by SCC must be in the SCC directory or subdirectories.