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 !!