Searching "roll up" on this site gives me only How do I convert Mathematica's suggestions to usable code which mentions the icon but says little about it.
I found nothing on it in the Mathematica documentation. Clicking a link for "Search all Mathematica sites" gave
Sorry, but no results matched for '"roll up inputs"'. Please try again.
I even searched the documentation for the phrase "combine several actions into one input expression" which is tied to this icon in one of the tutorials. But I cannot find a description of the use of the icon.
Where is it described?