It is that model in which there exists only one single class of jobs. In this type of models, all customers are indistinguishable with respect to their device service demands and routing behaviour.
Those models where the workload consists of several classes of jobs. Here customers may be partitioned into different classes. Each class has its unique service demands and routing behaviour.
It is the Queueing model of a system with an infinite customer population where customers arrive, receive service, and exit. The number of customers in the system varies dynamically. The number of possible system states is infinite.
It is the Queueing model of a system with a fixed finite number of customers. Customers arrive, circulate among the system resources, and exit after completely receiving service. The number of customers in the system at any time is fixed. Also, the number of possible system states is finite.
A mixed model can be viewed as a combination of two seperate submodels, a closed model and an open model, that share the common resources of a computer system. A mixed model with X classes of customers has Y closed classes and Z open classes, where X = Y + Z .