Society Central

I have clients looking at Premium Economy on United to Lisbon, return from Madrid. via Newark and IAD. My question is, because I rearely book United, on the flight from DEN-EWR and IAD-DEN, where do they sit on the aircraft, what class of service will they receive? Yes, I know they will have the Premium Economy on the transAtlantic flights....my question is what does United do to them domestic USA? NO, they are not stopping over, just making connections.