The 'Overall' Amplitude is the sum of all of the vibration energy occurring between 0 cpm and the data collector's maximum frequency (easily over 1,000,000 cpm). In other words, if you were to collect a spectrum over a frequency range of 0 - collector's Fmax and added all of the amplitude peaks together, that would be your 'overall' amplitude. Although not technically correct, that is one way to look at it.
Consider again collecting a spectrum from 0 - the collector's Fmax. Take every amplitude value, square it, add them together and take the square root of that sum. Although not exactly perfectly accurate, that is closer to what actually goes into calculating an 'overall' amplitude. That is known as a "Root Mean Square", or RMS, value.
All data is collected as an "RMS" value. That is an ISO convention and applies no matter what collector you may have.
The RMS value was settled on in order to guard against a transient spike in the signal distorting the true value significantly.
But there are several different 'signal detection' units available - RMS, peak and peak to peak.
How exactly are they related and calculated ?