On the same assembly line, one or more components of a product can be assembled. If only one model is assembled in the line, the production system is defined as a single model assembly line, otherwise it is defined as a mixed model assembly system. Assembly line balancing problems can be classified based on several characteristics such as the number of models produced on a given production line, the nature of activity times whether deterministic or probabilistic, and the nature or type of flow whether direct or probabilistic.