A bus is a grouping of your project objects for mixing and final output, which we detail in Managing Output. But, well before finishing your project, it is important to understand the role of different busses and how they work within your overall project structure.
Just as you need to define the structure for your project assets to manage them strategically, you also need to organize the output for your project. By grouping output busses together in a hierarchical structure called the Master-Mixer Hierarchy, you can define the relative properties, the States, RTPCs, as well as the Effects for the routing of your project.
You should spend some time considering how best to organize your project routing to streamline the mixing process. For example, you would simplify the mixing of your game audio by routing such sounds as ambient music or gunfire through corresponding busses.
To begin, you must understand the default structure in your Master-Mixer Hierarchy. It consists of master busses, each with their own role and hierarchy: