Map Index elements

I am a new MM user and have a question about the map index elements.
when I insert a *.htm link, the Microsoft edge symbol appears. so far so good.
if I insert another link, for example *.net, only a white field appears. With these links, if you right click on them, Manage Links, then an icon appears in the "Link Title" column. Can anyone tell me how this symbol is also displayed where I currently have a white field or the edge symbol for *.htm addresses? thanks to all helpers!