AM Demodulation with Envelope Detector
Without trying to over complicate things, an envelope detector is generally thought of as only applying to DSB and only when the modulation depth is less than 100%.

Basically if the shape of the modulating signal can be fully seen in the envelope of the modulated carrier, an envelope detector will be the simplest choice. 



Hopefully the picture above gives you the idea.

If the carrier is "over" modulated then the envelope of the modulated carrier no longer represents the modulating signal and this certainly applies to DSBSC and any type of single side band modulation.