In particular, where it says in that link;

"Credibility of N knowing M and K"

does that statement in the very narrow context of that link, and in all terms of mathematical expressions, mean exactly the same thing as:

"Probability of N knowing M and K"

Or is there a subtle difference in meaning that is such that the "Probability of N knowing M and K" would be expressed by a DIFFERENT equation (if any equation ) ?

and, if so, what is that subtle difference in meaning?

Is the word "Credibility" in that very narrow context of that narrow part of the link just another way of saying "Probability mass"? Because, and this is what has got me confused, if it is "Probability mass" then I can prove no such "Probability mass" with the particular equation they use can possibly exists! (proof far to complex long and arduous to show here so I won't)