Why splitting off the ASCS from PAS?
As you might know, currently when you will perform an installation of a fresh SAP system based on SAP NetWeaver 7.0x and higher, there is an instance for ASCS. It was separated from PAS for all the fresh installations.
But why SAP did that?
Because it is very crucial to ensure the availability of the Message Server and the Enqueue Replication Server. This way you will have one dedicated instance to perform the failover. Apart from High Availability, SAP recommends and/or wants to enable a setup where all systems have the same types of instances. For this reason and also to make High Availability enabling a bit easier, the split off functionality has been delivered.
But if you have installed your SAP System a long time ago and your ASCS is into the PAS, you can split them into 2 instances. For systems based on SAP NetWeaver 7.1 or higher, it is possible to use the latest version of SWPM (For further reference, please check SAP note #1680045).
I used as example the option to split off a SAP NetWeaver 7.3 EHP 1 with Oracle. But the option is available for every system and database combination present in the SWPM for NW 7.1 and higher.
But if you have a system based on SAP NetWeaver 7.0x, this option is not available in the 70SWPM, The only way to do that would be running a system copy of the system. Then during the target system installation, you would be able to install the new system with separated ASCS from PAS .
For further information regarding this subject, you can check the Installation Guide for SAP NetWeaver 7.1 and higher. Check the section 2.5 Central Services Instance for ABAP Separate from Existing Primary Application Server Instance of an Upgraded System for further information regarding the subject pointed in this post.
Also check the section 7.6 Splitting Off a Central Services Instance for ABAP from an Existing Primary Application Server Instance to know how to run the split.