In case a drawing contains objects that require extensions that are not currently installed, the drawing can still be opened. Objects not recognized by CET Designer will show once the drawing finishes loading, but they will display a texture to indicate missing data. Information about which extension is missing is presented.
When this occurs, you will see a yellow banner at the top of CET Designer that prompts you to remove unknown objects. If you do not want the objects removed but need to see them instead, you may need to install missing extensions which are shown in the tool tip.