Custom tools, toolboxes, tasks, workflows are created and stored in DGN libraries. When you open or create a DGN file you can see and use all the customizations in the configured DGN libraries. If the open DGN file is not a configured DGN library, you cannot use the features on the Tools tab in the Customize dialog while that DGN file is open. (The configuration variable MS_GUIDGNLIBLIST (set by default to point to MS_DGNLIBLIST and to the DGN libraries in the active interface component's folder) and the configuration variable _USTN_SYSTEM_GUIDGNLIBLIST specify which files can be used for customizations.)