Does "fiancee" even have any legal defintion in the US? Nobody signs anything when they propose and I'm pretty sure that states don't recognise it as a legal status, at least not until various common-law cohabitation conditions comes into play.
It's a particular type of immigration Visa the K-1 is a "Fiancee Visa". Note that you are required to get married within 90 days. The point of it is to allow US Citizens to get married within the US to non-US Citizens.
