I’m sure that a lot of you are already aware that you can drag a material straight from the Material Manager onto an object and that material will be assigned to that object. Did you know, you can use a similar technique to actually restrict to Selection Sets.
In Polygon Mode, first create the polygon selection.

Create a polygon selection
Next either drag the material from the Material Manager onto the selected polygons, or select the material and choose Apply from the Function menu.

Drag the material onto the polygon selection or choose Apply from the Function menu
Cinema4d will automatically create a Polygon Selection Tag and restrict the material to that selection.

Selection Set and Tag created automatically
Much quicker than creating the selection, choosing Set Selection, dragging your material onto the object, then dragging the tag into the texture tag . . . Now, all you need to do, is rename the selection set tag to something more meaningful. (That’s if you want to?)