In the exception of ad hominem circumstancial you said the argument seen as less of a fallacy and more of a legitimate motive .how ? Does it refers to the counter made by someone ? Or it is about the person who have vested interest ?

Let's look at the exception for the ad hominem (circumstantial) 

As the bias or conflict of interest becomes more relevant to the argument, usually signified by a lack of other evidence, the argument is seen as less of a fallacy and more as a legitimate motive.

First, keep in mind that how fallacious something might be is also determined by an expression of confidence. So virtually any claim or argument "you are biased therefore what you say is (definitely) wrong" is fallacious. However, like in law, motive does matter, and the stronger the motive, the more we can reasonably deduce that one's bias is getting in the way of facts and reason. For example, if all politicians who lose their elections from now claim that the election was "rigged," especially without evidence or evidence that does not hold up under legal scrutiny, then claiming that politician is most likely wrong in their claim of election fraud is perfectly reasonable and non fallacious, given the powerful motive and bias the politician has.