Optimality and efficiency are two measures that can be used by the system designer to select or compare between different optimal layouts. At the same time, the system designer has three choices of part assignment to cells in accordance with his needs (minimum sum of voids and /or exceptions). Unfortunately, the impact of choosing the type of part assignment on optimality and/or efficiency is not taken into consideration in any previous studies. In this paper, a critical analysis of the impact of part assignment in cellular manufacturing systems is elaborated on three cases borrowed from the literature. In the first case study, different grouping efficiency measures were used on two different optimal distributions having the same number of cells. These measures give different conflict evaluations, for that the designer’s decision to choose the optimal solution depends on the optimality rather than the efficiency. For the second case, the analysis was performed using three types of part assignment on the same optimal system (same number of cells). The results showed that the designer’s decisions depend on the constraints on the shop floor, since there is a conflict between the efficiency and the optimality. For the third case, one type of part assignment was executed on one system with different number of cells and the designer takes his decision based on the efficiency.