when booking the flight on United.com, and choosing the specific flight you can look at details and seat availability. I pick the flights that look good, but when I get to actually chose the seats they are not there. It is not a paying or status issue- we have 1k (and I even would pay if there was the choice). Also there are economy comfort seats available to chose from, but by far less than it showed available when browsing/choosing the flight and only undesireble ones, e.g. some middle seats spread throughout the economy comfort cabin