What's an efficiency dorm?

and why are they so cheap? I’m looking to house in Rubin in a efficiency room in studio, how are those?

It’s a low cost dorm, where they shove more people into the same size room (usually three people in a room designed really only for two)

But how are they? Is 3 people in a 2 person studio really that bad? Specifically the Rubin ones

It really varies, even within dorms. Some of the low cost triples in Rubin are massive, while others are teeny tiny and require bunked beds. It’s a bit of a lottery

Are efficiency rooms the only type of low cost dorms? How many people are usually in low cost dorms?