If you aren't sure whether the past or the present would be the correct one, or if neither feel right; like if it's made clear that they had an affair in the past, and we get lots of flashbacks and such, but they're also getting together again in the present time. Or if we first have the story of their past written out, and then we get to the present time and the fic keeps going. This is the ambiguous folder that can be used when neither the past nor the present folder feels quite right.