A single live fetal pole is referring to the baby in the stage when it is first visible inside the gestational sac. As for the intrauterine gestational sac, that is means the pregnancy is seen within the uterus and is not ectopic.

it disappears by the 81th gestational day

You will likely see the gestational sac and maybe the yolk sac.

What is a Blighted Ovum (Empty Gestational Sac)? A blighted ovum (empty gestational sac) occurs when a fertilized egg implants into the uterine wall, however, the fetus does not continue to develop past the sixth to eighth week of gestation. There is continued development of the placenta and amniotic sac which produces hCG (giving the positive pregnancy test). Most women either do not know they are pregnant or find out within the first trimester that they have a blighted ovum (empty gestational sac).

I believe it means that they have only seen one sac--as in one embryo...not twins.