no expert, but
I think Global MIDI channel simply means the channel your device is going to use to communicate with another device.
You probably know that the MIDI spec has 16 channels. No more.
So your device (one synthesizer) uses one of those 16 to communicate with other devices (i.e. a Sequencer, another Synthesizer, a MIDI controlled Mixer board).
The information flowing along your designated Global channel (let's say #3, for the heck of it), may need to speak to another machine for various reasons. Both machines need to agree upon one of the 16 channels to use. This is what is referred to as the Global channel.
If one machine is setup to use channel #3, and the other is using a different channel, then certain communication won't happen.
Also, if both machines are using channel #3 as Global, and the bass guitar track information is also on track/MIDI channel #3, then they will share MIDI #3. This is advised against by some manufacturers because it can cause miscommunication, and you may get unwanted results. So you are advised to use a separate and unused MIDI channel for Global, if you can, and put all your other instruments on unique channels.
By the way, the amount of Global information transmitted between machines, in other words, the Global chatter, is minimum.
The MIDI track data (information about the drums, guitar, bass guitar, etc.), will be far more data than the occasional chatter on the Global.
I think this is correct.
