I always learned that Open Bar = host pays, guests do not pay for any beverages (alcoholic or not). Open bar can be be just beer and wine (varieties of each), if the host chooses.
Full bar=all types of liquor available to guests. Also fully hosted by host, guests don't pay.
I see a lot of confusion on here re: open bar vs full bar.
It's good hosting not to have guests pay for drinks (open bar). It's ok not to have a full spectrum of liquor.
Correct me if I'm wrong.