The previous model is known as the envelope model and for each DMU k, allows us to find a whole set of other DMUs called benchmarks set of DMU k. The DMUs belonging to this set are efficient and have similar practices as the DMU k. The set of benchmarks for each DMU is found according to its projection on the efficient frontier. When a DMU is efficient, it is the only benchmark for itself. Mathematically, a DMU j is a benchmark for the DMU k when we run the model (I) for the DMU k and the variable j λ acquires non-null value.