User-1528094326 posted
The Rectangle is for formatting the runtime layout of the report. It keeps grouped data together within itself for whatever controls are placed inside of the rectangle. It is more or less a staticly sized container for formatting the report style.
A list is simply for repeating items per row or per object returned. It has no essentially containment, it will just repeat each object returned by the dataset its bound to in the format you have setup inside of the list object.