In MIDI instruments, drum sounds most often are separated by being placed on different keys, i.e. assigned to different MIDI note numbers. This means that when you use a keyboard to record a drum part in a sequencer, you usually use one key for bass drum, one for snare and so on. On many instruments used to play back drum sounds (drum machines, samplers and some synths) you can re-arrange the order in which the sounds of the single instruments are assigned to the keys. (Instead of assigning the bass drum to the key C1 you could for instance assign it to D1, or any other key available on your keyboard).
Unfortunately, most MIDI instrument manufacturers place their drum sounds on different keys and in different orders. This can be troublesome if you have made a drum pattern using one instrument, and then want to try it on another. When you switch instrument, it is very likely that your snare drum becomes a ride cymbal, or your hi-hat becomes a tom, etc, just because the drum sounds are distributed differently in the two instruments.
To solve this problem, and simplify several aspects of MIDI drum kits (like using drum sounds from different instruments, in the same “drum kit”), Cubase VST features a Track class called Drum Tracks. Parts on Drum Tracks are called Drum Parts.
The thing that separates Drum Tracks from MIDI Tracks is that everything played back from (or routed Thru) a Drum Track, is “filtered” through a Drum Map. Among other things, this Drum Map (described in detail on the next page) determines exactly which MIDI Note Number is sent out for each sound in your drum pattern (and thereby which drum sound is played in the receiving instrument). A solution to the problem above, would therefore be to make up Drum Maps for all your instruments. When you want to try your drum pattern on another instrument, you simply switch to the corresponding Drum Map as well, and your snare drum sound will remain a snare drum sound.