Kentico Specifying a Default Value for Document Type Attribute

If you want to insert some kind of default value in the form you can use macros to get the value.
You can first create a custom macro as per: devnet.kentico.com/docs/devguide/types_of_macros.htm#custom
And then use this macro in the default value field you highlighted in your screenshot like :

{#nameofmacro#}

Now when you try to insert new document you will see the value which the macro returns.