A couple of notes --

1) You may see F8 A8 on a route with a mix of 737-200 and MD-88 flights even though no one has reserved a seat in First Class. I suspect Delta is limiting the inventory of First Class seats to be the lesser of the two, namely the 8 F seats on a 737-200.

2) While it is rare, it is possible for a flight to have no possibility of carrying passenger who bought a T fare (e.g., VLD-ATL when there are no T fares anywhere from VLD). U5 T0 means that U seats were purchased by people to the point where its T subset was exhausted. It doesn't mean that Delta intentionally zeroed out T seats, and it doesn't mean that T fares were sold. T will go to zero all by itself with the purchase of higher fares.