Idly Rice

Idly rice is parboiled rice and used specifically for making idli and dosa.
Idly Rice goes through a single parboiling process, which means paddy grains are partially boiled prior to processing.

This reduces soaking time before grinding rice to make Idly & Dosa Batter. It is mushy when cooked and stays fresh & white for a long time. The cooked idly rice grains are smaller, softer and stickier than boiled rice grains

In conclusion, you can use idly rice and boiled rice interchangeably. You can use either rice to eat as a cooked grain or to make idlis. But you will get a chewier, sturdier grain with boiled rice. On the other hand, you will get softer, tastier idlis with idly rice.