Monte Carlo relies on data that describes the variation of elements within the system.  It also connects the elements such that their result is an estimate of performance.

For reliability modeling, this is easiest to imagine for a series system. For a system with two elements in series, a very simple reliability block diagram multiples the expected reliability for each block to determine the system reliability value. Yet, it is possible to have both elements at the low end of the range of possible reliability values, or the high end or a mix.