I worked in digital marketing a long time ago and I remember how Google analytics would add unique trackers to links in emails and social media pages. The ? and & are usually what is used. When the link is accessed, Google analytics stores the text that comes after the ? or &. The si stands for share info. But since there’s no text after, no info is shared. It’s like me giving you a blank piece of paper and telling you to save the information on it.