Hi @no_ming,
Expected exposure (EE) is the amount expected to be lost if the counterparty defaults. In other words, the EE will be greater than the Expected MTM, since it concerns only the positive MTM values.
Potential Future Exposure (PFE), on the other hand is the worst exposure we could have at a certain time in the future. In other words, PFE at a 95% confidence level will define an exposure that would be exceeded with a probability of no more than 5%.
While the definition of PFE is exactly the same as VAR, the notable difference in the context of credit exposure, is that it refers to a number that will be normally associated with a gain (exposure), whereas traditional VAR refers to a loss. VAR is trying to predict a worst-case loss, whereas PFE is actually predicting a worst-case gain.
Exposure E = Max(V,0) = Max(u + sigma*Z, 0)
Expected Exposure EE = average of only positive MTM's in the future
Potential Future Exposure (PFE) = u +sigma*NORMSINV(alpha)
Hope that helps!