The Assign Renderer rollout displays which renderers are assigned to the production and ActiveShade categories, as well as the sample slots in the Material Editor.
For each rendering category, the rollout shows the name of the renderer currently assigned, and a button that lets you change the assignment.
Click the button with the ellipsis to change the renderer assignment. The button displays a Choose Renderer dialog.
By default, the sample slot renderer is locked to be the same as the production renderer. You can turn off the lock button to assign a different renderer for sample slots.
Click to save the current renderer assignments as defaults, so they will be active the next time you restart 3ds Max.