From the manufacturer:

The EHX Intelligent Harmony Machine instantly creates matching harmonies to what you play. It’s like having one – or even two – guitarists jamming with you at the same time and always in perfect sync. The Intelligent Harmony Machine opens a door to the music of great multi-lead guitar bands and multi-part harmonized solos. Plus, its ability to apply harmonies ranging from simple to sophisticated will totally transform what you play. Of course, it also boasts EHX’s renowned impeccable tracking and genuine musical tone.

Select your key and the Intelligent Harmony Machines will generate single note, diatonic harmonies with a huge range of choices that includes both two- and three-part harmonies.