The simplified example does not do justice to a real guitar note, which is much less of a sine wave and more of a crooked sawtooth, at least in the simple case where only one note has been struck. If two or more strings have been played, both base frequencies and both sets of harmonics are present, and the supposed regularity gets really rough. Frequency and harmonics are very hard to extract useful info from, so almost all note processing goes for the amplitude envelope. Organ and synth makers long ago standardized on the definitions of attack, decay, sustain, and release to describe the loudness dynamics of notes. All of these things are characteristics of normal acoustic instruments.
Some instruments have almost no attack transient, like a softly bowed violin.
Some instruments have little or no release, well made electric guitars being one of them if you let the string simply ring until it dies out. However, on an acoustic guitar, even when you damp a ringing string, the sound it made reverberates inside the body cavity for a few milliseconds after the string stops moving, so there is a distinct release. The bottom part of the sketch shows the envelope of the note. This is a hypothetical outline just touching the peaks of all the signal vibrations. Strict musical theory says that the envelope is two sided, top and bottom, but we're interested in electronic processing, so I'm going to only work with one half.