A rule is basically an item path that locates one or multiple geometries or geometry components such as shading groups. Each rule has also a set of properties (more, are likely to come in the future) such as visibility state or material assignment.
Now let's say, you have in your scene an object named project://f40_layer0 and you would like to assign a material normal to all of its shading groups. To do that, you would just need to add a new rule in the shading layer. Set it to its item path and select the normal material.
project://f40_lwo_layer0 with a normal material applied
One fundamental thing to understand about shading layers is that rules act as overrides. Your geometries can have their materials defined via the material linker or via a Material Override, without being affected by a shading layer. If no rule affects a specific item, it's left unchanged.
Only the F40 glass shading group is affected by our shading layer.