The base directory for Phoenix.BuilderUIClient.UserFriendlyBuilderSessionInitialization.FilesToCopy.
Combining this with Phoenix.BuilderUIClient.UserFriendlyBuilderSessionInitialization.FilesToCopy should yield the full paths to the actual files to copy.
This field is only meaningful if Phoenix.BuilderUIClient.UserFriendlyBuilderSessionInitialization.Mode is Phoenix.BuilderUIClient.UserFriendlyBuilderSessionInitialization.InitializationOption.IMPORT_FILES.
Otherwise, it may be null, and should be ignored.