Let me explain this with an example. Consider the following code, here there are two paramaters passed, a and b. But only a is being used in the function.
F <- function(a,b)
So when we call the function with just one parameter it assumes that one parameter is the value for the variable that is being used. So if we call the function like this,
it assumes 2 to be a's value. Calling the function F with just one parameter doesn’t give any error.