Sf2 Player, or rather SoundFont2 Player, uses files called SoundFonts (.sf2).
There are no SoundFonts included in LMMS, but you can find a lot of SoundFonts on the internet, see 8.3 SoundFont resources. A SoundFont bank contains base samples in PCM format (similar to WAV files) that are mapped to sections on a musical keyboard. This is especially useful for people working with MIDI files, since most SoundFonts contain samples of real instruments. You can select the default SoundFont that will be used when loading MIDI files in the Settings.
Many SoundFonts have multiple patches which is multiple sounds, often different instruments or different styles of instruments. To select a patch you can use the patch and bank spinners, or click on the wrench icon to open a window where you can see all banks and patches.
Because the Sf2 Player is a MIDI bases instrument, you can't use ENV/LFO in it. This tab is intentionally left empty. You can also apply Reverb and Chorus directly in the plugin.
Some SoundFont files support reverb and chorus. SF2 player gives four knobs for controlling the effects:
The Size knob lets you select the room size for the reverb
The Damp knob controls the damping. Damping set to 1 will cut the reverb when the note is released.
The Width knob controls the stereo width
The Level knob controls the wet level
The Num knob controls the number of instruments you are supposed to hear. This represents how many signals are being processed.
The Level knob controls the amount of the effect you want. If it is set to 0 you only hear the original sound.
The Spd knob controls the speed/rate of the LFO
The Depth knob controls the depth of the modulation. Higher values increase detuning.