Number Card where type field is "Custom"

Hello Everyone,

I was making a Number card and I just found one custom option in the type field of the number card doctype. Now, the issue is where to write a server-side script and declare a whitelist function.

Do we need to write in sever-side scripting Doctype in API Type or do we need to write in the backend only of the particular doctype for which we are creating a Number card?

Where this custom in number card is used any use case if anyone has used it?

Below is the image.

Thanks In Advance !!

Both server script and backend whitelist function are worked.
You can add a number card to dashboard or workspace