MIDI is basically a “code”, which sends out a particular note as a signal that was originally designed to transfer music files easily. For example, C3 would be the key of C in the 3rd octave, making it easier to collaborate with artists by shooting MIDI data back and forth containing the components of a song. Recently MIDI messages have been interpreted to communicate with different functions, like Traktor which can take MIDI data to tell it what to do.
The reason I say this is, that’s why you get a note playing from your X1 when you press a button. It’s litterally sending out a signal that corresponds to a note like “F2”, which is then played by your DAW.
Getting back to the main question though, personally I use MIDI a WHOLE lot less than when I’m DJing. I haven’t worked with FL Studio in years, and am unsure of what it can do with MIDI. But there should be a way to map it out if you search through youtube, etc.
Honestly when producing I find that a solid keyboard and mouse is easier, but I’ve used MIDI maps for things like: Scrolling through drum samples, controlling volume parameters (makes mixdowns faster), as well as assigned knobs to a synth when I’m designing sounds/tweaking knobs.
Also, since you have Massive/FM8.. Massive has “macro controls” on the bottom, which are basically 8 knobs you can assign to anything on the synth, and can easily be assigned to a knob or fader on your X1.
Best piece of advice would be to sit down and try to figure out what you’d want to do with MIDI when you’re producing. For example if you find you always reach for a certain control, or need more control over a certain parameter like LFO, then assign it a MIDI control to help speed up your workflow. Then go create your own custom mapping.
If you don’t feel like doing this or are unfamiliar with custom mapping, you can probably find one online through google by searching the XI and FL Studio, then dissect it to see what they did and how it works so you can start customizing it for yourself.
Just experiment though! You can’t really break anything through MIDI mapping (knock on wood..), and any of the MIDI Engineers on here will be quick to tell you that once you figure out how to map it yourself through practice, it becomes addicting/fun, and your workflow will never be the same! (In a good way..)
Best of luck!