When you attach a fence & top on your Radial Arm Saw (RAS) for the 1st time, it takes a lot of time & effort. The top must be level, and co-planar with the blade in all directions. There is a lot of shimming and adjusting required. When this is done, the first cut (a shallow cut), marks a kerf on the fence as well as the top, from which all other cuts are referenced. Over time, this kerf is not sharp and smooth anymore which can compromise the accuracy of the cut. So it makes sense to route a shallow dado to accept an insert (I used 10mm hardboard because that is what I had available), as it saves time and wood replacing an insert instead of the whole top every time the saw chops it up. This idea is not mine (thanks Denis), but it is easy to do. I jammed the hardboard in the depth gauge of my router for depth of cut, and set up my shop-made dado jig for a 40mm wide cut. This 40mm width was not measured but gauged-I first ripped a few 40mm wide strips of hardboard on the bandsaw. It makes sense cutting a lot of extra strips for the inserts at this setting because the dado will be made according to this width! Then I pushed the dado jig against both sides of the strip to create the correct width. In other words the dado jig is set according to the actual insert that I will use, and not measured with a ruler.
Some photos (not the best quality as they are taken by my phone), might illustrate this better. The 1st photo shows the reserve inserts on the left hand side-enough for at least two world wars. Their edges are still furry as they haven't been sanded down yet.
I added another blade better suited for cross-cutting (way more teeth), as I only use the RAS for this purpose. With mitre cuts, the blade remains straight, and the would is angled against a jig.