For Open Ceiling configurations (Solatube 330 DS-O and 750 DS-O), the diffuser has notches that are designed to attach directly to an extension tube that is notched to match the diffusers. The top tube angle adapter, bottom tube angle adapter and the 0-90 degree angle adapters do not attached directly to an Open Ceiling diffuser as they are a slightly different diameter than the extension tubes and also are not pre-notched to accept the diffuser. 

For Closed Ceiling configurations (Solatube 330 DS-C and 750 DS-C), the diffuser attaches directly to the transition box. It cannot attach directly to the top tube angle adapter, bottom tube angle adapter, the 0-90 degree angle adapter, or an extension tube.