Net photosynthesis
The carbon a leaf fixes minus what it loses to respiration and photorespiration. It is the figure gas exchange studies report, and the one enrichment is meant to raise.
Gross photosynthesis is the total carbon a leaf captures. Net photosynthesis subtracts what the plant burns back off, through ordinary respiration and, in a C3 plant, through photorespiration. Net is the number that matters, because it is what is left over to build biomass.
It is usually written in µmol CO2 per square meter per second, the same units as PPFD but counting carbon dioxide molecules rather than photons.
Because photorespiration is subtracted out, anything that suppresses photorespiration raises net photosynthesis without making the enzyme itself any faster. That is the mechanism behind CO2 enrichment, and it is why the benefit is reported as a percentage lift in net photosynthesis rather than as a change to RuBisCO.
A rise in net photosynthesis does not automatically mean a rise in yield. The extra carbon still has to be partitioned to flower rather than to leaf and stem, which is a separate question from how much the canopy captured.