What SAP system ASCS, ERS, PAS, AAS is:
ASCS：ABAP Central services instance （Message server）
PAS： Primary application server instance （Dispatcher,dialog, batch, spool, or update)
ERS：Enqueue replication server instance （lock table）
AAS：Additional Application Server
Take a look at the data query works once the SAPGUI:
1, SAPGUI initiate SOCKET connection to the SAP server.
2, the reception it is ASCS the Message server.
3, Message server continues the connection is assigned to the PAS Dispatcher server.
4, Dispatcher server to find a Work processes (dialog) in the PAS.
5, then the user data in the ABAP layer operation, if there is data to lock table, the content is on the ERS.
HA installed features:
the ASCS small handles network connection, to put on the storage can be switched between hosts.
PAS is relatively large, to put on the store, you can switch between hosts.
ERS is very small, it can be clustered, two machines are placed, do not switch, so user data can be kept for.