USn is a useful distinction because apparently some Europeans use "Americans" to mean anyone who lives on the continent of North America. This is, of course, strictly true but in Canada American can only mean someone from the US. Therefore, if I use USn in a post it is clear I am talking about someone from the US, whereas I now know that if I write of an American the meaning is vague to someone from Europe, and probably doesn't have the specific meaning I intend it to. (This is something I didn't know before joining the Board.)