1K is the highest level of elite status that can be earned by flying a particular number of miles or segments. (There's a higher tier called Global Services, but that's invitation-only, based on some secret formula, probably involving some combination of annual spending and buying full-fare or premium cabin tickets.) UA doesn't have nice sensible names for its elite tiers, like silver, gold, platinum. Instead, the names are premier (2P), premier executive (1P), and premier executive 1K (1K). The abbreviations in the parentheses are both FT shorthand and internal UA computer codes.