It is used to handle the user requests that are coming from SAP GUI using DIAG (Dynamic Information Action Gateway) protocol on port 3200 + Instance Number.
(3201, 3202 etc)
Dispatcher manages all the work process and maintains a queue. Once the request is sent to dispatcher the dispatcher keeps the request in queue. Based on the availability of work process it will assign the process to the user request based on FIFO (First In First Out)
User Buffer - Once we logoff everything is rolled off.
Public Buffer - Until the instance is restarted