What does the seats that are "coasters" mean. how do they differ from a berkline? Is it the way the recliner operates?

Coaster is a name brand, just like Berkline is. Not a nickname or anything to do with any actual attribute of the chairs; just the company that makes them.

Coaster is not a name brand, and they do not make anything. They are the US distributor for the chairs made in China.