Psai's concept is based on the dynamic recombination of musical segments. These are automatically chosen and enqueued on-the-fly while your game progresses.
Each Segment contains a segment from the stereo (or surround) sum of your game soundtrack. This means there is only one main track playing at any time, and you don't have to care about multiple layers. The only exception of this are Highlight Layers. A Segment always belongs to exactly one Group, and a Group always belongs to exactly one Theme.