ArcGIS Mission Server is the ArcGIS Enterprise server that links Manager and Responder. It is optimized for communication and allows for peer-to-peer communication between mobile users while simultaneously enabling and updating the web application.
ArcGIS Mission Server facilitates communication by organizing the presence of mission members, populating the geolocation pings from Responder onto mission maps, forwarding message data between mission components, and more. All of these functions are done in real time or near real time to ensure a continuous flow of data between users.
ArcGIS Mission Server is unique among ArcGIS Server roles in that it does not contain a data store of its own. Optimized for communications, it only passes data between users as efficiently and rapidly as possible. This means that users in the Manager and Responder do not interact directly with the server.
For more information see, Introduction to ArcGIS Mission Server.