Round of drinks

A round of drinks is a set of alcoholic beverages purchased by one person in a group for that complete group. The purchaser buys the round of drinks as a single order at the bar. In many places it is customary for people to take turns buying rounds.

It is a nearly custom in Ireland, the United Kingdom, Canada, New Zealand, and Australia. In Australia and New Zealand it is referred to as shouting. This practice is also customary in many parts of North America, especially in areas where people with cultural roots in Ireland and the UK predominate. A notable exception was the UK State Management Scheme in which treating (i.e. buying a round) was forbidden, from July 1916 until June 1919.

In John O'Grady's They're a Weird Mob, Nino learns some customs related to shouting.

Your turn.

Your turn to shout

Because I shouted you.

He thought for a while and said, I get it. When you buy a bloke a beer, it's called a shout, see?

I haven't a clue, but that's what it's called. I shouted for you, now it's your turn to shout for me.

He looked quite stern, In this country, if you want to keep out of trouble, you always return a shout, see?

Bloody oath, it's the custom. Your turn.

In the culture of the United States Military, possession of a challenge coin can be used to determine who buys a round of drinks. One individual of a group lays down their coin, and all else present must lay down their coins as well. Anyone who does not have a coin with them must buy a round. If everyone can produce a coin, the challenger must buy a round.

...
Wikipedia