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 will show once the drawing finishes loading, but they will display with question marks to indicate missing data. Hover your cursor over an unknown object to display information about which extension is missing in the yellow tooltip.
When this occurs, you will see a yellow banner at the top of CET 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.