OK, let's start from the beginning.
When you play a note, there's "something" inside the DSPs that receives the note ON (and later the note OFF). This "something" is called in the Origin jargon a "Voice". The Voice contains modules which are connected together. You can see these modules and their connections by opening the "Edit" page. While the note is played we say that the Voice is "gated". When the note ON is received we say that the Voice is trigged ON. When the note OFF is received the Voice is trigged OFF. If you play 2 notes on a polyphonic program, then 2 Voices are "gated". If you play 3 notes then 3 Voices are "gated". It means that each Voice is generating the sound for 1 note. Modules within a Voice are called "VOICE" modules, and sometimes "POLY" modules because in a polyphonic program you'll get as many Voices as there are notes.
Inputs of modules within a Voice can be modulated by signals coming from modules inside and outside the Voice. Modules outside the Voice can be seen by opening the "Program" page. Sequencer, Galaxy, Env2D, etc... are such modules. Modules that you can tweak in the "program" page are called "PROG" modules and sometimes "MONO" modules. That's because they are shared by all the Voices.
Now back to the GATE/TRIG.
To drive a Voice we need a "GATE" (or - which is equivalent - a TRIG) signal which is specific to this Voice because that signal is just an image of the fact that one key is pressed. So if you play 3 notes, you'll need one "GATE/TRIG" for each note because these notes are never played simultaneously.
To drive the "PROG" modules we need another gate which the OR of the Voice "GATES/TRIGS". That "PROG GATE" is used to drive "PROG" modules that need it, for example the Sequencer.
Try a polyphonic program that uses the Sequencer. Play a note and keep it pressed. You will notice that the Sequencer starts when you play that first note. Then play a second note while keeping the first one ON. You will notice that the Sequencer has not been restarted at the beginning of the pattern. Now release the first note. The Sequencer continues to play as if nothing had happened. If you release the second note then the Sequencer stops.
That's where the Keytrigg and Keyboard modules show up (at last
)
The Keytrigg is a (should-be) hidden "PROG" module that manages the "PROG" gate.
The Keyboard is a (visible) "VOICE" module that manages - among other things - the "VOICEs" gates.
Because the "PROG" gate is intended to drive "PROG" modules it makes no sense to connect this gate to a "VOICE" module. So this connection should not appear in a connection dialog. This is what I tried to explain in the former post. Unfortunately, this connection still appears in the "-- MONO --" section of the connection popup menu. There's now a bug in our data base related to that so it will be fixed in the future.
The Keyboard "VOICE" module does not only manages the "VOICE" trig (output named TRIG). It also provides 4 "Keyboard Follow" (KF1 to KF4) and 4 "GATE/TRIG" outputs (Trig 1 to 4) which are related to the 4 keyboard follow outputs. Each KF has a lo/hi note setting. When a note is played within the range of a KF then the related "Trig" output is activated. You can use these "lo/hi range trigs" to control a drum sound for instance.
HTH