To keep track of all the Boos that you caught to get the "Gold Diamond". I missed one Boo and had no idea which room to find it, so I made a checklist with the essentials (the name, location, and area). Hopefully this will help you on your ghost catching quest.

Image used, Nintendo website: https://www.nintendo.com/en-za/Games/Nintendo-GameCube/Luigi-s-Mansion-268236.html