I take Forbes's point - if there is an independent listing of the part on IMDB, then it can be considered 'featured'. But I do think this is an issue at your discretion - you will know whether you feel the part in question is representative of you working to your best ability, and my feeling remains that, if you returned home from the shoot feeling you were underused, or asked to do something anyone could have done in your place, then you shouldn't list the credit, because its pointless. I know you might have worked with a big name director etc., but I reiterate, if you also know, that were someone to view the sequence in its entirety, they might justifiably say 'What, you mean you were the person with the big hat in the back of the crowd?' or 'that was the back of your head that I saw moving past Sean Bean sat on the park bench?' or whatever, then this is not a credit you need...it just isn't.
Forbes's case is frequently different, I believe, because as he says many of his 'supporting' roles end up having a vital part to play in the narrative. There is, in that sense, a distinction, and good 'featured' parts, especially if they reinforce industry awareness of your casting type, *can* be useful to credit, as can, of course, things like 'cameos' in adverts, about which there isn't the same stigma.
Another good way of defining the quality of the part is to ask whether you had to audition/recall etc. to get it - in which case, somebody was at least making a judgement call on your suitability and talent to a certain extent, as opposed to pure 'extras' work, where anyone can be pulled onto set regardless of their performing experience.
So put simply, it's always a judgement call in my opinion, as to whether you feel the part is worthy of consideration or not. If *you* felt it inconsequential, ditch it.