The Label Lines button overplots every line present in the displayed spectrum on the Main Plot Window. The default limiting flux is ph cm/s, but this can be changed to whatever limit the user prefers in the FilePreferences window.
The Label Peaks button first attempts to identify peaks in the observed spectrum, and then uses a simple algorithm that uses the location and estimated strength of the possible matching emission lines to label these features. It can fail if there are too many features (more than 50-100) present in a spectrum, in which case a message to this effect will be printed in the Text Output Window.
The Clear button will erase all labels from the plot.