The Cosmic Heart Compact was given by Neo-Queen Serenity to Usagi in order for her to transform into a much stronger and more evolved version of her standard Sailor form.
The Cosmic Heart Compact replaced the Crystal Star in act Act 26 when it was destroyed after the battle with Death Phantom. The compact first used in Act 27 when Usagi needed to transform when a Daimon appeared. The Cosmic Heart Compact has protected Usagi in her civilian form. For example from Cyprine brainwashing and it also created a floating barrier when the newly awakened Mistress 9 attacked Usagi.