What is the purpose of bulkheads in a liquid cargo tank?

49 CFR 383.119(d) (differences in tank vehicle type and construction); AAMVA CDL Manual Section 8.2.3, "Bulkheads," p.8-8-2

Recognizing that bulkheads create separate compartments, rather than just slowing surge, is the foundation for understanding the loading and unloading rules that follow. It also sets up the key distinction tested between bulkheads and baffles.