Question

How to set invoice type by default?

Answer

It can be done by modifying the Detect Invoice type script rule with the code below:

if( Context.Field("InvoiceType").IsVerified )
{
    return;
}
bool isCreditNote = false;
Context.Field("InvoiceType").Field("Invoice").Value = !isCreditNote;

To edit the rule, please go to the document definition, open InvoiceType field properties, select rule "Detect invoice type" - edit, open tab "rule Settings" and press edit:

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After saving changes and publishing the Document Definition, all invoice documents will be always identified as Invoices.