Renderer override allows you to use different settings for one or more rendering engines in different passes.
This gives an extraordinary flexibility when working with scene. You can use advantages of different engines in different parts of image rendering. For example render volumetric light with Scanline, main image with Vray, and trees with Corona, then reassemble everything on compositing.
It also gives ability to render some passes with high settings and reduce them in pass, where it is not required. Or, for example, enable or disable motion blur in different passes.
Current settings can be recorded in override via right click menu.
In one passer may contain only one Renderer override.