Particular/Form: Why do I see "3D Model for Use with Trapcode" when importing my OBJ file?

OBJ Files cannot be natively read by After Effects and need a plugin (like Trapcode) to read their information. The on-screen message is informing you that you need to use a plugin to display the 3D object and not just import it into the composition on its own.
 

• Use Particular, Form, or Mir with the OBJ file's layer set as the emitter type or base form.
  • If this fails to work or you get an OBJ importer error, try reinstalling these plugins.
• If you don't use these Red Giant plugins, you'll need to export or obtain your 3D Model in some other format.