The “icon” element points to a URL where an image can be found that can serve as an icon for the feed.
The icon element is intended to be used is intended to be used as an icon to be displayed along side your feed within your subscriber’s feed readers or aggregators. An icon is typically optimized to be displayed at a very small size, and typically has a 1:1 aspect ratio (an equal width and height). Whereas the logo element has no such restrictions (although it is recommended that a logo have an aspect ratio of 2:1). It is simply refers to an image associated with your feed.