On FatWatch’s weigh-in view, below the weight picker wheel, are two controls that allow you to annotate your weight entry for that day. You can set or unset a check and you can write a short note. The check and note appear in the Log view. In the Chart view, if a day is checked, its weight mark is highlighted in yellow.
I use the check to indicate which days I have exercised. I write a note if I did something unusual that day, like attended a friend’s wedding (and maybe had too much to eat at the reception).
I have heard of others using the note to keep track of body fat percentage, miles walked, meters swam, etc.
Are you using these fields? What do you use them for?