You can send messages to different channels like so:
/say message is Vicinity.
/o message is your Org (guild).
/t message is Team (active only when in a team).
Back before the new chat system, it was a pain to change chat channels; you could change channels by either clicking on the channel in your Friends drop down or by using /ch <channel name> like /ch vicinity or /ch vi and so on. That second method is even more cumbersome if you have a guild with a name like mine - /ch Devil`s Advocate is a lot to type. Personally, I still find changing channels with the newer chat system a little clumsy so what I did, before even the new chat system existed, was create some scripts for changing chat channels which I still use to this day:
/da changes to my guild channel using a script file named da containing /ch Devil`s Advocate
/tm goes to Team using a script file named tm containing /ch team
/vic goes to Vicinity using a script file named vic containing /ch vicinity
And you can make ones for whatever other channels you wish. It's handy to have scripts to change to shopping and private channels (like raid bots).
One thing that I added to these scripts is a message that only I can see confirming which channel I had switched to: /text message. So now, the scripts look something like this example:
/text Vicinity chat
The newer chat system displays the name of the chat channel you are in already so you might not find as useful anymore, but I still have it in my scripts and like the confirmation message (but probably more out of laziness).