What does this equation mean?

GCMs, when run with an interactive carbon cycle rather than a prescribed concentration, attempt to simulate $F_{\text{ocean}}$ and $F_{\text{land}}$ from first-principles biogeochemistry — ocean carbonate chemistry, marine biological production, terrestrial photosynthesis and respiration — so that the partitioning between atmosphere, ocean and land emerges from the model rather than being read off an ice core. This is the only one of the three approaches that can attribute the carbon budget to specific mechanisms rather than simply describing its net trajectory, but the terrestrial carbon cycle in particular remains one of the largest sources of spread across the CMIP6 model ensemble, meaning this mechanistic decomposition is exactly where the three approaches agree least.