I think it's worth getting a little deeper into what ullage is. The word crops up in several seemingly contradictory usages.
Capacity A tank has exactly one capacity. It is the volume from bone-dry empty until stuff starts to pour out of the vents. There is only one answer to the question what is this tank's capacity?
Ullage refers to measuring the distance down to the liquid surface. It starts from the top and measures down to the surface. The manual method uses a table to convert ullage to the amount of liquid present. The zero point is usually the completely empty tank—no heel. The volume indicated is called—wait for it—the ullage.
"Ullage" is a useful name for the empty space that often remains in a container of liquid after it has been filled as much as is practical.
Yes, and for any other state of filledness as well.
Sounding refers to measuring the filled space from the bottom upwards to the surface. The zero point for sounding is the point of loss of suction of the draft pipe or pump. This makes sense if it's your gas tank, and is the norm everywhere. The volume indicated is called the sounding.
So a full tank, a half tank, and an empty tank all have an ullage and a sounding. But if the ullage table is zeroed at a completely empty tank, you still have nonzero ullage at loss of suction. So when converted to liquid volume, ullage and sounding don't yield the same number.
Hopefully, this explains why a valid answer to what is the complement of ullage is—ullage. As already mentioned by others, it is used in brewing and distilling. It is also used this way in shipping. Heel would be problematic on ships. Another term that isn't exclusive to the pumped-down scenario, but is used for that situation, is ROB, which stands for Remaining On Board.