Mastotip: If you tag person x in a DM with person y, both x and y will be able to see what you said. A common workaround is to put spaces between the @ sign and the username (I.e. @ sandrockcstm @ orbitalstation.one).
This is really key if you are discussing a moderation related issue with an admin, say if person x is harassing you. If you tag that person you can inadvertently get them to see the conversation. This is not clear from the UI and a common trap for new users.
Some bloke who's managed to wangle a living writing about tech for DigitalTrends. My views are my own and all that.