In brief
Bulk fermentation is the fermentation of the whole dough before dividing and shaping.
Useful to know
Bulk fermentation is the fermentation of the whole dough before dividing and shaping. King Arthur connects this stage with gas production, flavour development and strengthening through any specified folds. [1]
It serves a different purpose from the final proof of an already shaped loaf. Mark the boundary between the stages when reading a recipe. Dividing the dough early for convenience changes the procedure. Follow the stated temperature and signs of readiness rather than carrying the bulk-fermentation time directly into the final proof.
Sources
For further detail and to check the information: