MCS stands for "Multilevel Concurrent Simulator." In order to better understand MCS, it is helpful to first understand its name. "Multilevel" refers to MCS' ability to simulate behavioral and gate level circuit models, and "Concurrent" means that the simulator can evaluate many different fault scenarios simultaneously. The foundation of this behavior is the Multiple List Traversal (MLT) algorithm and the concept of the Function List. MCS is also discrete-event simulator, meaning that it evaluates changes only at specified time instances (it does not look at changes in the circuit continuously as time passes, but rather only looks at the circuit at 2 ns, 4 ns, 6 ns, etc.)
MCS is the successor to Creator (the original simulator) and makes one large improvement over its predecessor, its multilevel capabilities. Creator is a discrete-event concurrent fault simulator like MCS, but is only able to perform single level evaluation.