The simplest topology is a bus consisting of a single server, as created if you install WebSphere ESB in a standalone server profile.
In a single-server SCA.SYSTEM bus, there is only the one server as a bus member. When you install a mediation module into WebSphere ESB, the queue destinations used by the module are defined on that bus member. These queue destinations are used by the SCA runtime exploited by the mediation module as a robust infrastructure to support asynchronous interactions between components and modules.
In a standalone profile, there can be only one messaging engine in a bus, a topology that is adequate for some applications.
A single messaging engine might not be adequate if the number of client connections becomes excessive, if the rate of message throughput cannot be sustained by the one messaging engine, of if the size of messages needs more space than the one messaging engine can provide in its data store.
To add more than one messaging engine to a service integration bus, you need to use a profile for a managed node in a deployment manager cell.