Basically you can do /gc (or any different command) and you will be in global chat. What you can do in global chat is view messages from everyone in global chat and send messages to everyone who is in global chat. Or you can do /sc (again, or any different command) and you will be in server chat. You can see messages from everyone on your server and send messages to everyone on your server. Note: If you are in global chat and someone on your server sends a server chat message you will still see it.
I think this feature is currently in use for admins and Dev's. I'm not sure if it's needed for people other than OP's.
We have sectioned off chat into 3 sections [main hub and freebuild] [training grounds all worlds] and [buildoff] we have done this so others do not get confused with what is being said and where. We may look into changing this choice later on but for now this is how we have chosen to set it up.
This suggestion was just to break up the chat in the training grounds for people who don't want the spam of all the different worlds. Edit: This suggestion may not be needed due to the fact that today is the release and so many people are on and chatting. The chat may get quieter as the server goes on.
The functionality is all there already - we have the ability to categorise channels as network-wide, shard-wide (a section of the network) or just local. Infact, you are all members of 3 default channels - for each category - just you don't have the permission nodes to utilise the commands to switch into or say in different channels yet.