It’s the way the radiator sections go together. They may be nippled together at both the top and bottom, or just at the bottom.
Older steam radiators have nipples across just the bottom portion of the sections. This is because steam is lighter than air. When the steam enters the bottom of a radiator (as it always will in a one-pipe steam radiator), it flows upward into the sections, displacing the air as it goes.
Hot water radiators, on the other hand, have nipples across both the upper and lower portion of the radiator sections. Even though hot water rises, it doesn’t move as quickly as steam. The double set of nipples encourages better circulation of the hot water across the entire radiator and leads to greater efficiency.
Around 1905, when two-pipe steam became popular, contractors began to use hot-water radiators on steam systems. The old steam radiators with their single set of bottom nipples quickly faded and became obsolete.