|ABAP Central Services Instance (BC-CST)|
AS ABAP instance of an SAP system containing the enqueue server and the message server (ASCS = ABAP SAP Central Services). There can only be one such instance in the SAP system, and it can be made into a high availability SPOF (single point of failure) instance.
The enqueue replication server (ERS instance) is set up for this.
Alternatively, you can work on an an ABAP system with the classical central instance (containing the enqueue work process - usually the message server is is also running on this system).