Hey, you are correct in your guesses so far. Presets can come from different places, and the letter prefixed to the preset indicates where it came from and also whether it can be changed or overwritten.
F stands for Factory Preset and it is bundled into the app bundle on OSX and iOS and is in the application directory on Windows. Factory Presets cannot be changed or deleted but only copied.
U stands for user preset and is saved under ~/Library/H9 Control/Presets on OSX and iOS and AppData/Roaming/H9 Control/Presets on Windows. User presets can be changed and deleted.
S stands for stompbox and indicates that the preset came from an H9. These can be changed or overwritten but not deleted.
And L stands for list and indicates that a preset is saved inside a list, which are saved under ~/Library/H9 Control/PresetLists on OSX and iOS and AppData/Roaming/H9 Control/PresetLists on Windows. These can also be changed or overwritten but not deleted.