United Airlines to introduce tiered fare categories for premium cabins  ReutersUnited to Offer No-Frills Fares in Its Premium Cabins  The New York TimesUnited unveils basic Polaris business fare in premium cabin overhaul  CNBCUnited is rolling out 'basic business class' to make premium flying cheaper. Here's how it will work.  Business InsiderUnited Airlines splits business class into tiers on long-haul flights  Axios