The Composition relationship indicates that an object consists of a number of other objects.
The interpretation of a Composition relationship is that the whole or part of the source element is composed of the whole of the target element.
A specific kind of server can be modeled as a node composed of a device and system software implying an existence dependency between individual servers of that kind and the individual devices and system software instances of which they consist.
A Composition relationship is always allowed between two instances of the same element type.
An Application Component may be composed of two or more sub-components.