The smooth line and envelope are used to detect outlier observations in an ObsWin, using the Lightcurve Workbench tool.  Outlier observations are light curve artefacts resulting from atmospheric conditions (clouds entering the imaging field), equipment issues (telescope mount tracking problem, image focusing failure) and so on. Peranso allows to automatically detect and deactivate outliers. This is described in full detail in Tutorial 16.

Light curve of T Aur. The thick blue line is the smooth line. Above and below the smooth line are thin lines, marking the envelope region. Observations outside of the envelope are considered outliers and are drawn in bright yellow. They can subsequently be deactivated to exclude them from period analysis calculations.