What does the idiom "backroom deal" refer to?

Which of the following scenarios best illustrates a "backroom deal"?

A company announcing a new product at a press conference

 A public voting process for a new community project

Business leaders privately negotiating a contract behind closed doors

In which context is the idiom "backroom deal" most often used?

What is a common connotation of a "backroom deal"?

Which sentence uses the idiom "backroom deal" correctly?

 The chef made a backroom deal with the customers by showing them the kitchen.

The politicians made a backroom deal to pass the law without public knowledge.

The artist made a backroom deal by displaying his paintings in a gallery.