Meaning of `envelope` in context of time series

While checking TIMESAT, more specifically
http://web.nateko.lu.se/timesat/docs/TIMESAT33_SoftwareManual.pdf page 50, section 9.4 TSM_GUI.

Then I read:

"The fits are affected by a number of options for detecting spikes, adapting to the upper envelope..."

What's the meaning of envelope?

In physics and engineering, the envelope of an oscillating signal is a smooth curve outlining its extremes. The envelope thus generalizes the concept of a constant amplitude. The envelope function may be a function of time, space, angle, or indeed of any variable.

Time Series without envelope

Time Series with upper and lower envelope

Note: Images and more information can also be found here