It's a common idiom to use "over" as an equivalent for "divided by". I'm not certain that's what's meant here, now that you question it, but I assume it means "

It's a common idiom to use "over" as an equivalent for "divided by". I'm not certain that's what's meant here, now that you question it, but I assume it means "give the volume in fractional liters". EDIT: Actually, looking at the context again, I'm quite certain that's not what is meant at all. They must want the part of the volume exceeding a liter, as mentioned in other comments. But that was definitely not my first interpretation, due to the idiom I just described.