MIDI Message
By default your Zendrum transmits MIDI Note ON/OFF messages when you strike triggers, and Control Change messages when you interact with any additional controls you have (such as the sustain button).
Your Zendrum is also capable of transmitting nearly every other standardized MIDI message type.
Before continuing, please bear the following in mind.
1) There are good reasons why Zendrum Corp. choose not to expose these features. MIDI can be a tough language for the layman and it's very easy to become confused by the myriad of MIDI message types available.
2) Not all sound modules or MIDI interfaces may support all message types. In many cases they simply are not applicable to drum machines.
3) The implementation of some messages is not complete within the Zendrum or is otherwise crippled by the lack of data entry features.
In short, messing around in this area is not for the faint of heart.
NOTE: Your Zendrum has limited support for all MIDI messages types except for After Touch. If you require After Touch, consider using Channel Pressure instead.
A brief overview of the standard MIDI messages types:
Channel Pressure
After Touch allows instruments that implement it to detect when you are still exerting pressure on the key after Note ON has occurred but before Note OFF has been transmitted (i.e. you are holding down a key on the keyboard and then start pressing harder/softer). Obviously this doesn't really have much application in regards to your Zendrum whose triggers are simple piezo-electric transducers and cannot detect such pressure. Still, you may be able to find a creative use as it is a MIDI message that your Zendrum is capable of transmitting.
Channel Pressure is similar to After Touch. Whereas After Touch messages occur for individual keys (notes), Channel Pressure affects any notes played on a specific MIDI channel.
Control Change (CC)
Sets a particular controller's value. A controller is any switch, slider, knob, etc, that implements some function (usually) other than sounding or stopping notes (i.e., which are the jobs of the Note On and Note Off messages respectively). There are 128 possible controllers on a MIDI device. These are numbered from 0 to 127. Some of these controller numbers are assigned to particular hardware controls on a MIDI device. For example, controller #64 is Sustain.
The notepickers in ZenEdit will display the names of common controller values (such as Sustain) to save you from having to look them up.
Note ON/OFF
The most basic of MIDI messages. This is the default message for all the physical triggers on your Zendrum. When you strike a trigger pad, a Note ON message is sent indicating both the note number and the velocity (force) that it was struck with. Immediately after Note ON is sent, a Note OFF message is also transmitted. By varying the time that elapses between the ON and OFF messages you can control the note duration. NOTE, your sustain switch is not capable of transmitting Note ON/OFF messages, regardless of the trigger type setting.
Pitch Wheel
Any control on your Zendrum is capable of transmitting Pitch Bend messages, though to get full use of it you'll probably want to have a wheel or knob type control installed, or use a foot pedal. Note that the note values and crossfade settings are not used for Pitch Bend messages.
Program Change
These cause the MIDI device to change to a particular program (sometimes also referred to as patch, preset, instrument, etc).
Please note that you cannot control which program is being selected (it is based on the velocity you used). Useful only if you need to detect the presence of any program changes rather than a particular one.