If you select some text then use CTRL+D, the editor will search the next match in the file and add it to your selection, and whenever you type something both of the selected segments of text will be edited in the same way - you can extend the selections with SHIFT+LEFT and SHIFT+RIGHT.
It’s hard to explain in an intuitive way, but you’ll get it if you try it.
Another simpler example is CTRL+SHIFT+UP and CTRL+SHIFT+DOWN: your current selection splits to the next line in either direction.
Something similar happens with CTRL+SHIFT+MOUSE_LEFT.