Consumption bar vs package bar
A package bar charges a fixed price per guest per hour, however much people drink. A consumption bar charges for what is poured, so a light-drinking crowd usually costs less on consumption and a heavy-drinking crowd costs less on a package.
How bartenders track consumption
The bar counts full and opened bottles at the start and end of service, or rings every drink into the register. The count is checked with the client or the event lead at close.
What happens when the bar hits its cap?
Hosts often set a spending cap. When the bar reaches it, the bartender tells the event lead, and the bar switches to a cash bar or closes, as the BEO says.