Some instruments sound a different note to the one that you play. For example, most trumpets and clarinets will sound a B♭ when you play a C on them. Some other instruments play the same note at a different octave. For example, most piccolos sound one octave above the note played. And some instruments do both. These are called instrument transpositions. Because PlayPerfect listens to the sounds you play, it needs to know what the transposition of your instrument is, or it will tell you that you are playing the notes wrong when you are actually playing them correctly.
Here are some common instruments and their transpositions: