The target of the bombing raid on Siegborg, 6 March, (M/Y) was the railroad marshalling yard. undet of pff means the results were undetermined by the PFF, or lead aircraft.
PFF was abreviation for Pathfinder Force. The PFF ship was the lead ship in the bomber formation that had the precision radar target locating system know as GEE. Gee was the code name given to a radio navigation system used by the Royal Air Force during World War II. It measured the time delay between two radio signals to produce a "fix". It was the first hyperbolic navigation system to be used operationally.
The only way to find out the number of planes in the raid is to get the actual mission 189 reports of the 4 squadrons composing the 397th Group.
On 6 March, the the 1st Infantry Division was about 3 miles from Bonn and the germans were retreating across the Rhine ( the Bonn bridge was blown on the 8th) . The railyards in Siegborg were a legitimate military target at that time. The abby may have been a secondary target or target of opportunity because of possible use as an observation point.