zulip/help/include/how-to-reply-intro.md

Many chat apps have prominent “reply” or “reply in thread” buttons. These buttons are necessary, because it's often hard to figure out what conversation messages belong to if you don't use them.

When you start composing a message in Zulip, it will automatically be addressed to the conversation thread you're reading (unless you are starting a new thread). Because everything is organized into threads, it'll almost always be clear what you're responding to. This means there is no need to repeat what has already been said when you reply.

You can still quote part of an older message for reference, or forward a message to another thread.