Having a chat module is crucial for modern web applications. your client wants to develop a Chat service where his internal employees can have discussions. Unlike, Normal Group or Private chat, user can create a Thread for a Particular message and start a conversation there. Threads also can have a sub thread of conversations. Chat Messages are paginated and grouped by Date/Time.
Build a Chat Application with above problem scenario. you can use web sockets or socket.io to develop the real time chat and design a system.