C.Fn IV-5 Focusing Screen

Can anyone tell me why a Custon Function has to be set if a focusing screen is changed?

I ask because I fitted a new Ef-D gridded screen to my 50d and set the Custom Function C.Fn IV-5 to Ef-D

The reason for this post is, what setting to use for a DIY gridded screen.

After removing the old screen from my 50d, I now had a spare sitting in the box.

It didn't take me long to realise, if I added grid lines to it with a pencil, I could use it in my 40d.

Now my delema. On my 40d, Do I set C.Fn IV-5 to Ef-D or leave it on the default Ef-A?

What difference does it make?

The camera needs to know so it can meter the scene correctly.

I suppose settings for your custom screen might take some fooling around with to find the best option.

That's the bit I don't understand. Why a few lines etched on the screen alter the ability to meter the scene.

I don't know what has changed but back when I used medium format it didn't matter what was on the focusing screen.