Also, Is there a proper name for this strip? Is there any documentation on what other types of icons appear here?
Please let me know if there is any more information you might need that would be diagnostically helpful
It's to help highlight a line that's been navigated to, due to actions in another VS panel.
For example, if you did a "Find All" and then clicked on one of the results, it would navigate to the match, highlight the line (yours looks to be doing it purple), and also apply that marker to help (further) visually enhance the line you are presumably looking for.