There really isn't much of an official distinction going on between extras and those with one line, it all depends on the person's individual contract and will vary from production company to production company. On a film set, the guy in the corner who says "Aye sir!" would be considered an extra by the entire crew.
I'm not gonna passionately get in to an argument that Sulu is in fact NOT an extra but actually a "bit part" or whatever. The point is that he was in no way a main character and got less material than the one shot guest stars of the week would get. I'll change my argument to being that he was little more than an extra if we can get remotely back on topic.