The F1 means that there is still 1 First Class seat available for sale. The E0 menans there are no upgrades to first class available for discounted coach fares. If the F1 does not get sold it can be used to upgrade at time of departure.

On U's 757, they have reduced First Class from 24 to 8, but since all the planes are not yet reconfigured, they will book the flight based on the lower number and fill up first later.

7 is the maximum any bucket will show and they are not necessarily added together.