So, when it comes to register/bookmark/rectangle/etc commands, I tend to forget which keys run which commands. They all use C-x r as a prefix, which makes it all the more easily forgettable. Luckily, emacs is (as always) one step ahead of me. Typing C-h after a prefix will show you all bound keys/functions for that prefix. So, typing C-x r C-h shows me all the C-x r keybindings available and what they do.