The short answer
Double steam rice is steamed twice before it is milled. This sets the starch inside the grain, so it cooks firm, rarely breaks and stays separate even through a long dum. It also reheats well, which makes it popular with caterers.
Steam vs sella vs raw
| Type | Processing | Texture |
|---|---|---|
| Raw (white) | Milled directly | Softest, most aromatic |
| Steam / double steam | Lightly steamed before milling | Firm, separate |
| Sella | Fully parboiled in the husk | Firmest, golden |
When to choose it
Choose double steam when the rice has to wait: biryani for a dinner that runs late, food for a function, or leftovers you plan to reheat. It gives you most of basmati's look with far less risk of a soggy pot.