Conventional optimization technique suggests that marginal unit generation sensitivity (MUGS) may be calculated based on perturbation at optimality. The calculated MUGS however only applies to the perturbed operating point. Often times it is not advisable to apply this local information to predict generations at another loading level with considerable load change, and therefore another calculation has to be performed to obtain the generation and its sensitivity at that loading level. It is of high interest to obtain a global pattern of the sensitivity in a wider range of loading levels, which bears great potential in applications such as congestion prediction and LMP calculation. Existing work have shown MUGS can be well approximated by a linear function of load. In this paper, explicit formulations are derived for some special cases and show that MUGS is precisely a linear function or constant in those cases. The usefulness of MUGS is demonstrated with two applications, congestion prediction and LMP (sensitivity) calculation. Based on MUGS, optimal load shift factor (OLSF) is proposed to facilitate predicting future binding constraints such as transmission congestions. As a function of MUGS, LMP and its sensitivity can be easily obtained.