If you write a review, make a comment, tag or other type of content, use the My Shelves feature, rate something, or create lists within BiblioCommons, that information is retained on BiblioCommons’ servers. You can choose to make your My Lists and My Shelves private.
If you plan to add ratings or comments to things you’ve borrowed, you should make them shared. Otherwise, no one else will be able to see what you’ve contributed! Regardless of your default shelf setting, you can make individual items private or shared.