We can add notes or descriptions for projects, but nothing for actual designs, which to me is actually more important.
Due to the timeline system used, it's not always practical to change something, then rewind to where you made the change and reverse it, then move forward again. Even with good build practices it's difficult to do, so you make a copy and make changes, but there's no easy way to know what was done. For instance, I make revisions and label them as such, but unless I name the design "Arm version 2, increased input shaft diameter by .02mm, lengthened by 1mm and shifted left 3mm" I have no way of knowing what was done to it compared to version 1 without keeping external notes or a mile long file name. Since Tinkercad is designed to be web based, you might not always have access to your revision notes.