/new is new chat. New is used for newer players to ask for help or for new players to simply chat in. New players are put into this chat by default when they first join the server.
/help or /hc or /h is help chat, the primary chat to be used when asking for help. If you need to find something out, this is the best chat to ask.
/local or /l is local chat, which can be used for speaking to small groups in your immediate area. The range of this chat is only 64 blocks, so is ideal for chatting semi-privately, away from everyone else on the server.
/yell or /y is yell chat, used similarly to local chat, but for an even wider range of 256 blocks.
/general or /g is general chat, used for talking about pretty much anything, but only over the particular server. If used in Horizon, only people in Horizon can see it, if used in wastelands, only people in wastelands can see it, etc..
/global or /gl is global chat, which is nearly identical to general chat, being used for just about anything. But instead of being for a particular server, everyone can view the chat from everywhere.
/offtopic or /ot is off topic chat, which can be used for anything totally off topic. Ideal for stuff not related to anything in the server or Minecraft.
/market is market chat, used for buying and selling goods or services.
/recruitment or /recruit or /rc is recruitment chat, which is used for recruiting players to either towns or nations.
/veteran or /vc is veteran chat. It serves as a general-purpose chat but is exclusive to Aetherseeker rank (850 hours) and above. Any players below the appropriate rank can not see what is said in this chat.
/event or /ec is event chat, used for hosting or broadcasting events being held on the server. Could be used for any competitions or celebrations.
/pvp is pvp chat, which can be used to challenge players to fight.
/town or /tc is town chat and is used for speaking to individuals in your town. It’s a good way of communicating with everyone in the town at once.
/nc is nation chat, used for communicating to everyone in the nation at once.