I was messing around with my dash and after holding both buttons I was presented with 3 numbers that I could change. It wasn't the time and no other details about what the numbers represent was given. It was almost like it was a PIN input or something to enter another menu. I left the numbers alone but am curious what this menu was.

It's a 2013 CForce 800.

Another odd screen that isn't listed in the manual is a screen that lets you adjust the screen brightness. With this is some random numbers and letters. Any idea what these numbers and letters mean?

3 or 4? digit is to adjust the speed/odometer.

It's 3 digits

Here's a picture, the number is 620, however the first number is blinking so it didn't show in the image.

The other number I'm wondering about is "L16H":

3 or 4? digit is to adjust the speed/odometer.

Thank you, this was a huge help. My dealer didn't know this either, now we both know

For people that look at this the default values are:

Cforce 800 = 529
Cforce 500HO = 705

The other screen "L16H" I just figured out, it's for "LIGHt". This is the screen that lets you adjust the light level or brightness of the dash. This isn't in the owner's or service manual.

The 3 digit number is for the speedometer correction, bigger wheels = bigger circumference = higher number

It's 3 digits

Here's a picture, the number is 620, however the first number is blinking so it didn't show in the image.

0n my Cforce 550 when you press both buttons at the same time you get those three digit numbers. Those are to adjust the remaining km to next service interval. If it says 620 then your next service will be in 620km from that moment. You can adjust it from i believe 001 to 999. when you pass that 620km the oils can symbol flashes on the dashboard telling you that its time to change oil or simply telling you that you need to have your service interval now. Thats on my Cforce 550, I believe it is the same on the Cf800 too. I am not sure if you can make adjustments of tyre sizes on the dashboard but as of my experience from my Cforce 550, those three digit numbers indicates remaining km to your next service interval.

On my CFORCE 550 when you press both buttons simultaneously you enter setup mode. If you change the 3 digit number to 000 you can change the odometer reading back to 0 miles.

On my force 800 ATV one day after mud trail the odometer reading went wild going up in miles reading while stopped. I tried everything to stop it without success. After washing it it stopped. But now my new 100 mile ATV reads 2500 miles.
I tried going to 000 code to reset the odometer but on the cf800 it did not work. Anybody knows how hot do it?

[2015 Z Force 800 53/Trail EPS]

On my CFORCE 550 when you press both buttons simultaneously you enter setup mode. If you change the 3 digit number to 000 you can change the odometer reading back to 0 miles.

That actually should be the speedometer calibration settings. No one should be able to reset the odometer.

[2015 Z Force 800 53/Trail EPS]

On my force 800 ATV one day after mud trail the odometer reading went wild going up in miles reading while stopped. I tried everything to stop it without success. After washing it it stopped. But now my new 100 mile ATV reads 2500 miles.
I tried going to 000 code to reset the odometer but on the cf800 it did not work. Anybody knows how hot do it?

Are you super-sure you are not looking at one of the trip meter readouts?

I am absolutely sure it is the odometer. I am an engineer and I know what I am saying. But the issue is I would like to reset the CF800 back to the original mileage. The issue is how? On the 450L is a piece of cake but on the CF800 it does not work. What I don't understand is in the first place how the malfunction occured.

It occurred when the calibration code was changed, it has nothing to do with service intervals as someone mentioned earlier that it does.