Hi.
I went to Options/Reading and appearance/Channels to disable some of the intense PR noise.
What is the difference between System messages, Public chat, Notifications and Global messages?

Hello,
I can answer only about public chats and global messages.
The first one should be quite clear, it is all chats on a table.
Global messages are main room messages, such as people creating, leaving and joining public tables. It is also unfortunately messages about people suddenly disconnecting/reconnecting.

I have experimented a bit with notifications recently, and found that for some reason this also includes the message when pressing W, or messages in some games, like C in Duck racing, but not all, so I am not myself sure about what all is a part of that channel.

Hello,

System messages are important notifications about the functioning of the playroom, temporary issues, general messages from admin, and such. For example, reboot notifications are system messages.
You probably don't want to miss them, even though they are frequently repeated when something is getting wrong, as they can often affect you a lot.

Public chat is what it says: it's all chat messages posted by other users, publicly on a table. Private messages go on their own channel.

Notifications are messages that require your attention immediately, and most often which you also should react quickly to as well. Basically, it's when you are asked a question or required to do something right now.
For example, "It's your turn" is such a notification.

Global messages are general information about people entering or leaving a table or the playroom. These ones are less important, and probably those are you may want to disable in certain circunstances.
Note that, messages related to the table you are currently in go in another dedicated channel called table messages.

thanks Aminiel.