The Drum Kit Voice is a special Voice. Unlike the so-called "Normal" Voice, the Drum Kit Voice has just one Waveform assigned per key - and a different instrument, or articulation of the percussion instrument, is assigned to each key between C0 and C6. If a Drum Key is a multi-sample, most often the samples are mapped "vertically" and you reach the different samples by velocity range. A Drum Kit VOICE has 73 Elements (Waveforms), a Normal VOICE has 8 Elements (Waveforms); but each can occupy a PART of either a PERFORMANCE or MIXING setup. And it can be assigned a DUAL INSERTION EFFECT. This is assigned to the PART as a COMMON parameter, although routing to the Insertion Effect is handle on a per Drum KEY basis. this is true because unlike a piano, where you might want all the keys to share the same effect, a drum kit is really 73 instruments - each with its own routing and needs (you might want a certain effect for just the Kick drum, and not the cymbals).

The Effect routing is per drum. A drum (Key) can be routed to the System Effects (reverb and chorus processors) or it can be routed to one of two Insertion Effects selected for the kit. When a drum is routed via the INSERTION EFFECT OUTPUT it is removed from the System Effects. When a drum is routed to an Insertion Effect it will arrive at the main drum kit output but it will have the insertion effect present (applied). Any two Insertion Effects can be assigned to a Drum Kit Voice (found in [COMMON EDIT] > [F6] EFFECT). They are treated in parallel or routed from A-to-B or B-to-A. "Parallel" means each is separate and a drum can go through one or the other. The A-to-B and B-to-A routings are what is called "in series" (one after the other).

For a detailed article on assigning and routing within a Drum Kit please see these two articles in the RESOURCES section here on Yamaha Synth:

Building a Custom Drum Kit

and

What is a Drum Voice?

If you get stuck or have specific questions after reading through these short articles, please post back here.