Engine vibration monitor

[sire]

On the 737, what does the engine vibration monitor monitor?
And why does it go from zero to 4.0 and not from 0-10 or zero to 100%?
Do the numbers represent anything? I remember reading about MILS somewhere.
Yes, trivia on those long legs with no real answers.
Ppruners, please help.

[EEngr]

For what it's worth, DDG found this thread:
https://www.pprune.org/tech-log/9221-b737-400-ng-engine-vibration-indication.html

[sire]

Thanks, at least it’s something.