Chat

On civ servers, chat is managed by the CivChat2 plugin (source). Chat messages are not global by default, in contrast to most minecraft servers. Instead, messages are sent to the local chat, which is only visible to people within a certain radius. Messages can also be sent to Namelayer groups, which bypasses the distance restriction.

Mechanics
When sending a chat message to the local group (the default), only users within a certain radius of you will be able to see the message. This radius differs from server to server, but is generally around 1000 blocks. You can increase your range by sending messages from a high y level - again, the exact values depend on the server, but you generally get around 1 extra block of broadcasting range for every block you climb above y 100.

You can also switch your chat group from local chat to a Namelayer group with the  command. Messages sent to a Namelayer group will be visible to anybody in that group with the read messages permission, regardless of how far away they are.

Direct messages will also bypass the distance restriction.

Global Chat Group
Some civ servers have a Namelayer group for the sole purpose of providing a global chat. This group is usually called  or. It is used to effectively bypass the local chat restrictions and send messages which are visible to anybody, regardless of distance.

In earlier civ servers such as Devoted, this global chat group was entirely player-owned and managed, requiring players to join the group manually before being able to talk in it. Some later civ servers like CivClassic cemented the global chat group as a gameplay mechanic, automatically adding new players as members to the group and having the group be admin owned. However, not all civ servers do this.