AT1 is an 'autotuner', normally used to correct the pitch of a voice singing (slightly) out of tune. Compared to 'Autotalent' it provides an improved pitch estimation algorithm, and much cleaner resampling.
AT1 does not include formant correction, so it should be used to correct small errors only and not to really transpose a song. The 'expected' pitch can be controlled by Midi (via Jack only), or be a fixed set of notes. AT1 can probably be used on some instruments as well, but is primarily designed to cover the vocal range. It's also usable as a quick and dirty guitar tuner.