Turbine Vibration Units


I don't "get" how vibration works and hope someone can chime in.

I've encountered three common units for sensing the magnitude of rotor vibration: "IPS" (inches per second), "MILS" (?), and "UNITS". The first two can be found even on the same engine, IPS for N1, MILS for N2, and then then UNITS seems to be some kind of normalized value for use on the flight deck displays.

How does vibration work? One unit seems to be distance-per-time and the other is a mystery. How are the vibrations normalized to go from 0 to 5 (ish)?

Alls I know is I read the gauge and swag what it says and put it on the trend log.
Challenger. CF34 MILS.