When should I use "Missed Previous Fill-up"?

Hello, I am just getting my data into the app as I've previously used other apps to track my fuel consumption, but now I am in a dillema. Since all my fill-ups are.. previous, should I be using this function? I do have exact dates and all that and have already properly inputted all of them into the app, it's just that I'm unsure if I should check this or not / when I should be using it, like only if I miss a fill-up between other fill-ups? Also, fill-up = full tank, right? Sorry for the confusion and thank you in advance.