Originally, the C15 only supported Sounds, where all voices are controlled by the same set of parameters. Since version 20-28, Sounds and Sounds are available consisting of two (, ). Each Part can use up to 12 voices based on its own parameter set - so most parameters will be available twice, per Part.
In Sounds () the Parts are assigned to two separate keyboard ranges. Part I can be played between the lowest key (C0) and the Split Point, Part II between the Split Point and the highest key (C6). The Parts have separate voice allocations, which means that they can have different settings for Mono and Unison. The Split Point is not affected by the Shift function, the Master Tune or the Part Tune.
In Sounds () each voice of Part I will be played simultaneously with the corresponding voice of Part II. Both Parts share the same voice allocation, which means that they have identical settings for Mono and Unison.
Note that held Keys will reset when loading whole Presets or Parts of different types.