AudioUnits is the new plug-in format that Apple developed for CoreAudio under OS X. Any manufacturer can adopt this format, but they are not required to (for instance many applications use VST under OS X as well). AudioUnits is now the Plug-in format used by Emagic’s Logic Audio when used under OS X; it remains to be seen what system other developers adopt. AudioUnits allows for full two way communication between the host environment and the plug-ins, allowing for Virtual instruments such as soft synths to be able to receive MIDI commands and return audio, or sending audio into a plug-in to be processed and the output to return to the host environment. Apple includes a few AudioUnits in OS X 10.2 in the way of a Sample Rate Converter, Reverb, and a Virtual Instrument compatible with the DLS Downloadable Sounds format.