Modulate something, and it changes a bit. Mod is short for modulation.
Modulation in guitar etc. effects include chorus, flange, vibrato and phase. All of which adapt the original sound, with delay and pitch changes, usually working in tandem, to produce those various effects. Distortion, overdrive, et al, are not included in 'modulation'. Neither are the ...
It's for modulation effects.
From the Spark User Manual, page 3:
(H) Effects Control
Adjusts the amount of Modulation, Delay, and Reverb in the current preset.
Use the SPARK app to change the effects type and detailed effects settings.
This is consistent with other amps.
On Fender's Mustang V.2, the MOD knob is for setting modulation effects levels.
To paraphrase this post: https://www.vguitarforums.com/smf/index.php?topic=22478.msg200736#msg200736 Nothing is impossible, but it's cheaper to buy an amp with actual effects loop.
If you have access to another amp and speaker you can use take signal for the looper out of the headphone out, as this article suggests: https://www.zombieguitar.com/using-a-...
Try some positions near the speaker. If you use a 3mm cable to plug the amp's headphone output it will sound like Line without Impulse Response, unless you use an IR simulator pedal like Nux Solid Studio.
For an android phone: Plug a 3mm male to male cord in the headphone out on the amp. Plug the other end of the male to male 3mm into the headphone side of a 3mm audio splitter. Open the recording software that comes with your phone. Plug in the male jack from the splitter (need a 3 ring male) into the phone. The recorder will say it will be using an external ...