When the expected number of client requests to the AS Java is large, you may need to resize your cluster by installing new Java Instances.
You install additional Java instances on different physical machines, thus adding more hardware resources and improving the failover capabilities of your system.
Adding Java instances is performed during the installation of AS Java.
More information: Installation Guide for the respective version of your SAP NetWeaver system at service.sap.com/instguides.
Scaling the cluster by adding a Java instance