It's a test tone generator and as audio effect you can put it into any audio track and blend it with the original auduio. And its absolutely great in doing that.
But you also can use it as a limited monophonic synthesizer (which helds notes until the next MIDI Note-On message) since it allows MIDI data to be fed into it. [Tip: You can use Melda's own and also free 'MUtility' plugin with the option 'Silence when Playback is OFF' to shut the audio.]
You can create so many different wave formes and shapes, it's unbelievable.
A shame that Melda didn't upgraded it with polyphonic input, MIDI-Note-Off sensivity and maybe some LFOs to control some parameters, creating a great basic synthesizer out of this one.