Why do some have an F and others have a D?
Those letters indicate the wake turbulence category of the aircraft.
Based on context—
LFA, callsign "BUCKY," is a flight school and is unlikely to by flying "Upper Large" aircraft—the categorization being used in the video is probably the "Wake Turbulence Recategorization" (RECAT) rules, which is an older version of of the wake recategorization project (see FAA JO 7110.659C, the latest version of RECAT in use before its cancellation). The current standard is known as "Consolidated Wake Turbulence" (CWT) and procedures are defined in FAA JO 7110.126. The wake turbulence category of a particular aircraft type can be found in FAA JO 7360.1. If you had taken the screenshot at a slightly different time the datablock would have shown the aircraft type in the righthand side instead of the groundspeed.
Note also that this is NOT a depiction of a real-world FAA system; VASAviation feeds ADS-B data into their own simulator software. It looks pretty close but you cannot assume it is the same thing that the controllers were seeing at the time.