Instances 
Definition
SAP R/3 instances are defined during the installation of the SAP R/3 System.
The term “instance” is often used synonymously with “server” or “application server” in the SAP R/3 System. This depends on the selected installation environment. You can install multiple instances on one application server.
An SAP R/3 instance defines a group of resources such as memory, work processes, and so on, usually for a single application server or database server within an SAP R/3 client/server environment. Application servers share the same memory areas and are controlled by the same dispatcher process.
An SAP R/3 instance is handled by CCMS as a unit. A user logs on to the SAP R/3 System using an instance.
An SAP R/3 System can consist of one or more instances, such as an SAP R/3 System with its own instance with only one central server or a client/server system with two or more separate instances.
The following applies for every SAP R/3 instance:
· Separate directories are defined on the UNIX, AS/400 or Microsoft Windows NT server, on which the instance is to run
· It can use shared file systems
· Entries are created in operating system configuration files (/etc/services, /etc/sapconfig...)
· Communication entries are created in the host
· Start and system profiles are created
· SAP R/3 operating system users are installed
An SAP R/3 instance is not assigned automatically to an operation mode.
See also:
Creating the Instance Definition for All Servers
Prerequisites for Starting Remote Instances
Creating the Instance Definition for a Server
Starting and
Stopping Instances
Monitoring Servers and Work Processes